Mercari complaint: label problem with USPS

My complaint:

I am a GREAT customer. I sell mostly tee shirts and short on here and other apps. I have perfect feedback.
I use USPS for label. You offer a 6.00 label and the next level is 12.00!!!! IT does not cost 6 to mail these tees but it actually is 8.00. I cannot charge them 12! So I am stuck with the 6 option. About half of the time my stuff either gets returned and I have to add a bunch of stamps to the mercari label OR the buyer has to pay 2.00 at the post office to pick it up!
EVERY time buyer contacts me saying I owe them 2.00, I try to get their info to pay them the 2.00 I owe bc the mercari postage options are not right. Each time, instead of trying to help me, Mercari sent me awful messages about conducting business elsewhere! I would not owe these people money if the label was correct! Now I just got a FINAL WARNING????? from Mercar bc I have yet another person that i owe postage money. What am I supposed to do? THe label is wrong. The buyer wants their money back. Instead of sending me warnings why not make an 8.00 option for USPS since that is what it costs? Not 12, and not 6!
PLEASE remove the warnings from my account as the reason I got placed in this situation where I owe money to people is because your system will not produce a correct label. It is impossible to make it happen unless you add an 8.00 option to USPS.
I did nothing wrong.
I am trying to fix your label mistake. Often it goes undetected but when they see the error at the PO, it falls on the buyer. They need to be reimbursed somehow at least until you change the price of the label or make an 8 option.
In the meantime, I work super hard to be the best seller that I can. I ship right away and I am honest and accurate with my items I sell. PLEASE take away these awful hateful warnings that you gave me because I was forced into having to pay the difference re your error in the cost to get the stuff moved. This is EXTREMELY unfair. My account should not have any kind of final warnings lol. It makes me sick as hard as I work to keep my business great on your site. You will not find a better seller. Horrible treatment by you.

Suggested solution:

Make an $8 label option for USPS
Remove the warnings on mikewasko7@gmail.com that you sent because I was forced into figuring out how to pay the difference when the $6 label was not enough and 12 too much. Extremely unfair practice.