OP - You didn't specify for how long your rental was and that of course makes a difference as the charge is per day. A quick check of the Avis site suggests that Avis charges $13.99 per day per seat without any discounts, so presume that you are renting 2 seats for five days?

Problem is that most states prohibit the resale of child safety equiptment and the second there is a scratch in a piece of it, it goes in the trash, so the reality is that most of these seats don't see many days use before they go in the trash. Thus, there rental is expensive. Avis is, of course, buying in bulk so it's not paying anywhere close to retail.

You didn't provide your location in your profile, but presuming that you have US-compliant seats at home, I would bring them along. Gate check them just to keep tabs on them. Many retailers, Amazon for one, have $10 protective bags which are good to keep the seats clean in an aircraft hold (won't stop them from being tossed around).