Essentially using the Priority Mailbox to secure an item inside the poly mailer. This will be shipped as Ground Advantage.

Absolutely not @usa_vintage_goods . Priority boxes can only be used when you pay Priority postage rates. On top of that, "flat rate" Priority boxes require labels for Priority Flat Rate services.

Can I use a Priority Mail Box inside a blank poly mailer and ship Ground Advantage? | Shipping

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Can I use a Priority Mail Box inside a blank poly mailer and ship Ground Advantage? | Shipping | eBay Community