If there is a return address, then you will probably get a bill for underpaid mail, which is the postage plus a $1.50 processing fee. (I don't know if multiple items will result in multiple processing fees, though).

If there is no return address, the recipient will be asked to pay before collecting the package. There have also been a few instances where the recipient was asked to pay even though there was a return name and address on the package, so informing your buyers is probably a good idea, just so they're not hit with a surprise and can let you know if they receive the bill.