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First Class Mail is generally used for personalized correspondence. By Rule all invoices, statements or documents specific to the individual is 1st Class mail. Rates for first class mail begin at $0.55 for the first ounce and $0.15 for every ounce there after. Postage meter users pay $0.50 for the first ounce.
First Class can also be grouped into presort to receive a discount. First Class Presort can be as low as $0.383 for up to 3.5 ounces.
Retail provides for packages from 1 oz to 13 0z. However if you have a system that will create a 4 x 6 address label you can qualify for First Class Commercial Based pricing up to 15.9 ounces.
Priority Retail is for packages or envelopes over 13 oz. This requires no special prep and can be placed in the standard Mail stream.
Commercial Based Pricing is available to users who can produce a proper 4x6 barcoded address label. Commercial Based Pricing provides the added benefit of tracking tied to the address. It also provides a 10 to 25% discount on the rates.
The USPS created a category for Marketing Mail. This class was formerly known as Third Class or Standard Mail. Marketing mail provides a substantial discount to First class mail but also has its own rules.
1) Must be at least 200 pieces
2) Must be marketing or generic in nature
3) May travel at a slower rate. (always travels via ground while 1st class travels via air)
4) There are several classes including (letter, Large envelope, and parcel).