Many mailers choose to presort their mail because it results in lower postage. Because you're doing some of the work, it costs the Postal Service less money to process and deliver those pieces of mail. We pass that savings on to you in the form of commercial postage prices. Basically, "presorting" means grouping mail by ZIP Code. All of the pieces going to the same destination get grouped into the same bundle or tray. You’ll sort to specific areas, and then work your way up to more general areas. All leftover mailpieces are combined together.
Some beginning mailers choose not to presort their mail; instead, they pay the full First-Class Mail price, put stamps on their mailpieces, and drop them in a collection box. That's easy! Other mailers don't want to presort their mail but still want to get the lower postage prices. To do that, they use a presort bureau or letter shop to prepare their mail.