Why am I committed to buy items when I request seller to combine shipping and they don't?

[sandscape530]

I put 3 stamps in my cart. The shipping for each was US2.70. Since they obviously will be mailed in one envelope, I requested an invoice with combined shipping from the seller (which other sellers have provided me). I got an automated message saying the seller does not combine shipping. I was unwilling to purchase the stamps with 3 separate shipping costs, but the cart wouldn't allow me to remove the items. Suddenly, because I asked the seller a shipping cost question, it said I committed to buy the items.  I went ahead and paid because it looks like I have no option, but why can't that information allow me to remove the items? Obviously, next time I need to contact the seller first to see if they combine shipping, but I still think it's unfair to the buyer to let them ask, be denied, and the force the sale. Almost $8 to ship three small postage stamps is absurd.