3 different advertisers requires mass customization => extra overhead. You'll get to choose advertiser + t-shirt combos; not sure if added flexibility will make up for the increase in price, but I'll keep it in mind!
As an advertiser, I would definitely like to be able to be allowed to A/B different designs for the ads, it sounds pretty simple for an advertiser to be able to provide a few different designs and let people choose from them, or are you referring to a different cost?