Why pieces per route changes the price
Most presort levels reward sorting mail down to a ZIP Code or a facility. Carrier route pricing rewards something different: how many pieces you deliver on a single letter carrier's route. The more of one carrier's route a mailing covers, the less handling each piece costs USPS, and the lower the per-piece price.
That produces a result that surprises most mailers. Postage per piece is not fixed. The same 40,000 names can price several different ways depending on how they distribute across carrier routes, and a list that concentrates into fewer routes can cost less to mail than a list of the same size scattered across many. Density, not just quantity, drives the price.
The carrier route price tiers
USPS Marketing Mail and Nonprofit Marketing Mail carrier route prices step down as the pieces per route increase. The commonly used thresholds are:
- Carrier Route Basic: at least 10 pieces on the route. Flats only; see Letters and flats are treated differently.
- High Density: at least 125 pieces on the route.
- High Density Plus: at least 300 pieces on the route.
- Saturation: coverage of the great majority of the route's delivery addresses rather than a flat piece count. Saturation is measured against active residential or total delivery addresses on the route, so it is reached by covering the route rather than by adding a fixed number of pieces.
A route that fails the lowest carrier route threshold does not lose all discounts. Its pieces fall back to automation or other presort pricing, which is higher than carrier route pricing but still well below single-piece. See What are the presort sort levels.
Each route qualifies on its own. One mailing routinely contains routes at several different tiers at once, which is why a single blended cost per piece can hide what is actually happening underneath.
Letters and flats are treated differently
Flats have a Carrier Route Basic price. Letters do not. For letters, carrier route pricing begins at the High Density level, so a letter route holding fewer than the High Density minimum prices as automation rather than as a lower carrier route tier. This matters when deciding whether adding pieces to a sparse route is worth anything: for flats it can lift the route from automation to Carrier Route Basic, and for letters it cannot.
Routes that just miss
Real lists rarely land cleanly on the thresholds. A typical list has routes sitting a few pieces under a tier: 118 pieces where 125 would qualify, or 7 pieces where 10 would. Those routes are paying a higher per-piece price for every piece on them, not just for the missing few.
This is worth checking before a mailing drops, because the fix is small. Filling a route from 118 to 125 is seven addresses. Those seven pieces cost postage to mail, but their arrival can reprice all 125 pieces on that route at the lower tier. Whether that trade comes out ahead depends on the gap between the two prices and on how many pieces are already on the route.
Whether filling a route actually pays
The arithmetic is worth stating plainly, because it does not always favor filling.
Adding pieces to a short route produces a saving on the pieces already there (they reprice at the lower tier) and a cost for the pieces added (they are new mail, and they carry printing and production cost as well as postage). The mailing comes out ahead only when the saving on existing pieces exceeds the cost of the added ones.
That balance is friendliest on routes that are close to the line and already hold many pieces, because a large existing count reprices on the back of very few added pieces. It is least favorable on routes far from the threshold, where many pieces must be added to reprice comparatively few. A route 40 pieces short is a much weaker candidate than a route 5 pieces short, and it is entirely possible for filling a route to cost more than it saves.
Two costs are easy to leave out of the comparison and should not be: the production cost of the added pieces, and the value of the added recipients themselves. On many mailings the second point reverses the decision. Filling short routes adds households in neighborhoods the mailer already chose to reach, so even a break-even or slightly negative postage result can be worth it for the extra coverage. For political and nonprofit mail this is often the main reason to do it rather than a side benefit.
How short routes get filled
Filling a route requires additional deliverable addresses that fall on that specific route. They can come from a suppressed or held-back segment of the mailer's own file, from a previously unused portion of a purchased list, or from a new list order targeted to the relevant ZIP Codes. See House vs acquired lists.
The addresses have to be selected by carrier route, not by ZIP Code. A ZIP Code usually contains many carrier routes, so a block of names from the right ZIP Code will scatter across routes and mostly land where it is not needed. Useful route filling means matching each candidate address to its carrier route first, then taking only the ones that land on the routes being filled, and stopping at the threshold rather than overshooting it.
Overshooting is not harmful to qualification, but it is wasted spend: pieces added past the threshold earn no further discount until the next tier.
What to ask before a mailing drops
- How do the pieces distribute across carrier routes, and how many routes sit just below a tier?
- What would each of those routes need to qualify?
- What does the mailing cost as submitted, and what would it cost with those routes filled?
- Is there a source of additional addresses on those specific routes?
- Do the added recipients have value beyond the postage math?
A list that has been standardized and matched to carrier routes can answer all of these before anything prints. See CASS and ZIP+4 and How many records do I need for bulk prices.