Keep in mind that if you print a mailing label on PayPal for first class mail, your customer might receive a postage due notice. Here's why:

PayPal automatically prints a delivery confirmation.
Delivery confirmation is permitted for first class mail only if the envelope/package width exceeds 3/4 inches, or for any size priority mail package.
If you have delivery confirmation on a thin first class envelope, some post offices will boost the piece up to the priority mail rate (they can't add width, so they add a "service" to your mailings to make it conform to the rules). Accordingly, the recipient will have to pay the difference between your postage and the applicable priority mail rate, which is generally just under $5.

There was a thread on this issue last year after a Chip Board contributor dealt with the post office on this matter. Robert - you may remember it, since you pointed out the post office's rules (I was shocked to learn of the width requirement, but I verified it in the postal regs)