Is this considered poshmark mail fraud?

If anyone doesn't know what "poshmark" is, it's an iPhone app specifically for selling and buying fashion. If I buy an item, I have to pay an additional $7 just for shipping. The buyer gets an email from poshmark with the prepaid postage to my address. If she then CANCELS the order on the app, I get my money back, yet she still has that pre paid shipping label. If she does happen to mail it, using that shipping label, will that be considered mail fraud? And what if I do the same exact thing, for my buyers so they don't get charged? Is that mail fraud?

If you don't pay for a service you receive yes it is fraud and theft.