Ebay Postage Bags

Hi All, 

Where can i get these ebay postage bags from? Are these the bags i am suppose to use for the printable label? Are these bags available at the post office? 

 

I noticed it says the postage cost of $6.95 will be taken from my account at the end of the month so does this mean, once i get the bags i just put them in the mail box with the postage label on them? 

 

I have my own packaging (bubble wrapped postage bags from kmart that cost me $2 for the pack) Could i use the postage label on this or is this only for the ebay bags? Should i do flat rate and select a price of my own for these ones i got from kmart?

 

Any help would be great!!! So confused!! 

 

Thank you!