PayPal users can not be upcharged ;-) and paypal charges occur the second you send your order thus entering into an agreement on the price stated at time of purchase. Further that - texas law has a that covers these orders stating that suit is possible if orders are wrongfully canceled due to misleading facts presented at time of transaction:

http://law.onecle.com/texas/occupations/101.201.00.html

So PayPal regulations + Texas Law SHOULD cover any and all Texas purchases. They can "ask" you to uppay or cancel your order, but you have to authorize the canceled order unless they assume "out of stock" and cancel all orders past a certain point legaly due to stock availability, not sure how that could fly but I imagine it will cover them