Use Pirateship.com for first class parcel, about $2.89 if I recall right with the discount. No fees to you for membership or anything like that. More on reddit.com

16 oz or less can ship First Class Package ($2-4) 16 oz+ can ship via Priority Mail ($8-10) And if the item fits, you can use Flat Rate Priority Envelopes ($7.33) More on reddit.com

Cheapest I get for the same zone priority mail is $44.80, or $36.58 (w/$100 insurance) on PirateShip . Cross country (west coast to east coast), it's $99.30 for priority on USPS, $80.54 on PirateShip. More on reddit.com

USPS does have a postage calculator that you can put destination, weight, dimensions, etc. into, and it will give you a price. However, the USPS retail prices are very high regardless of the service you use, so I would recommend using a shipping site like Pirate Ship . They have contracts with USPS (and UPS) that let them sell labels at greatly discounted prices. Pirate Ship also has a free shipping calculator that gives you discounted rates with both custom-sized boxes and flat-rate boxes. The cheapest service to ship your package with is almost definitely Ground Advantage, however it is slower than Priority Mail (which the flat-rate box would use). If you want to ship with Priority Mail, and can fit your items into the Medium Flat Rate Box (11inx8.5inx5.5in), it might be a few dollars cheaper than using a custom sized box. However, it really does depend on where you're shipping to. Feel free to check out the calculator, and let me know if you have any more questions! More on reddit.com