Prices Changed Twice

This is the 2nd time I've screenshot the discounted prices as I'm checking out and then seen a discrepancy when order is processed and I was charged a higher amount. The 1st time I returned everything and they were very apologetic and said it was a website error. This definitely doesn't seem like an error anymore and I question how many times I've been upcharged after the fact. I've also had items that were available for return change to being final sale after purchase. How are they getting away with this?