If she special-ordered the shirts, and agreed to the price, that is pretty much a verbal agreement to pay for them. I work for a company that does some promotional items like shirts, etc. and if it's something that we can't re-sell, we have to eat the cost if the customer doesn't pay. Also, not paying for the shirts will give her a bad rep with this company....they will not let her order again and they may even take legal action to collect the money. It also makes greeks look bad...when we do stuff for GLOs, we make them pay up-front because college students are notorious for ordering things and not wanting to pay for them.
Wishin, the only suggestion I have is that the owner might be willing to let you pay for 1/2 the cost now and then wait to pick up the shirts...you could pay the other 1/2 when you pick them up. It sounds like she gave you a pretty good deal on the shirts, so I doubt you could get an additional discount.
Good luck.