Express Mail is the only USPS service which bears a time guarantee (or your money back). Typically, overnight service is possible, but some destinations only bear a 2-day guarantee. The flat rate for Express Mail, which requires a special envelope, is currently $18.95. Within New England, it may be less expensive not to use a flat rate envelope if your mail is only a few ounces (as low as $12.95), but to most other destinations, the flat rate envelope is usually your best option for price. To determine the USPS's guarantee to a particular ZIP code, please see this link: http://webapps.usps.com/expressmailcommitments/landing.jsp
Priority Mail is typically delivered within 2-3 days (3-4 days outside the continental U. S.) and ships by air. This is not a guaranteed service and occasionally takes longer, but it typically operates within this time frame. Flat rate shipping is available using specially provided envelopes and is currently priced as follows: $5.15 for small envelope, $5.30 for large envelope, $5.35 for small box, $11.35 for medium box (both shapes), and $15.45 for large box (both shapes). Large flat rate boxes shipped to APO ZIP codes automatically receive a $3 discount. Non-flat rate Priority packaging (boxes and evelopes) is also available, or you may provide your own packaging. The postage for these shipments is determined by both the weight or your mail and the distance to its destination. Therefore, fairly light shipments within New England are typically cheaper when you pay by weight, but heavier packages traveling further are often better off with Flat Rate prices. Priority Mail has a fairly low size threshold before additional charges are levied.
Please note that because Express Mail and Priority Mail both travel by air, additional restrictions may apply with regard to potentially hazardous contents (flammable liquids, for instance) versus ground-based shipments.
Parcel Post is the cheapest and slowest option for making domestic shipments. Parcel Post travels by ground and typically takes 7-10 days for delivery outside the northeast, although it can occasionally take up to 21 days. (This is rare.) For large shipments traveling long distances, Parcel Post prices are typically a little over half of Priority Mail prices, and Parcel Post shipments can be quite large before additional charges are levied due to their size.