I am using latest version of zencart 1.3.9

I am selling an item 13" x 13" x 1" (vinyl records), which weigh 1.2 pounds. want user to select from USPS media mail or Priority mail in usa. What comes up as cost for priority mail is the usps priority small flat rate box price (which i presume is because it is light enough, but obviously won't squeeze into that little box). the cost of flat rate box is $4.95, where the option i need is plain Priority mail which is $8.10.

anyone have an idea what setting I can make and where so this price is offered instead?

Thanks in advance for your help.
Betsy