It technically is illegal, if you go through the process of ordering supplies via the USPS website or if read the fine print on the boxes.
Re Paypal: if you are a business or premier account, sending money is still free, but they charge you to receive payments, regardless of how it is paid. The rate begins at 2.9% plus 30 cents, I believe, on any payment you receive [which is reasonable, considering you can now accept credit cards, and it costs a lot more than that to get a credit card machine and merchant account with Visa or MC]. Also there can be charges on getting your money out, depending on how you do it--bank transfer, using Paypal debit card, or sending money from your PP balance isfree; getting a check sent to you costs 1.50, I think.