If you try to order more of an item than is available, you will get a little message above the order form, and the quantity will not increase.
Priority Mail is just one set fee, no matter how many items you purchase.
The charges displayed on your shopping bag are calculated based on the information supplied but they make be modified when you checkout if there is a discrepancy. For example, if you do not indicate California residence on your shopping bag, no state sales tax will be added. When you check out, however, if you order address is in California, the tax will be calculated and added at that time.