The USPS shipping will not upgrade to Regular Flate Shipping

Hi;

I noticed even if I have two T-shirts in my cart the USPS calculates the shipping for small flat rate priority mail instead of regular flat rate priority. I have adjusted the dimensions of the t-shirts to wide verity of sizes with no resolution to the problem. I really appreciate any help with this.
Here is the link to the site: http://www.papercuties.com

There is no support for volume calculations in USPS, so if you select a rate with the USPS settings, it will always be available. If you are just using the flat rate boxes, you could use Volume-Based Shipping (www.getclearthinking.com/volume-based-shipping) to set up some volume-weight rates that are based on the box sizes.

Thank you, I am going to look in to that.