Can I use paypal shipping on flat rate envelopes?

I want to use pay pal shipping since it seems cheaper than regular shipping for the things I sell on ebay. I use flat rate boxes and envelopes. Can tape the paypal shipping label on flat rate envelopes?

And if I do this will pay pal still charge me 2.9% of my total sale (plus the .30) cents?
Update: Ebay already charges me 9% whether I use their postage thing or not =( Altogether Ebay and paypal charge 11.9% plus 30 cents per transaction and that percentage includes shipping which is why I dont even do .99 cent listings anymore.
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