jackal

This.

It's not intentional--it's a function of the way their computer systems do rate analysis of the competition in conjunction with forecasted fleet availability. Note that Thrifty's primary competition, Enterprise (which Thrifty competes most strongly with and sets their rates in accordance with), has a similar rate for both sets of dates.

Once you book a particular rate plan, though, you are locked into that rate plan regardless of the length of rental UNLESS you do something that invalidates the rate plan (such as pick up too early or return too late for a weekend rate, etc.).

That said, I can't guarantee this will be the case 100%. You can often see some more detailed rate information in travel agency GDS systems. A good way to get access to this is to do a test booking on Travelocity and then click the "Rules & Information" tab, which shows the full T&C as published in the GDS. In your case, the rate rules for your 8-day September rental come up as this:

For comparison, here are the rules of the higher 7-day rate:

Now, I believe those are just the rate rules that Thrifty's reservation engine uses to figure out which rate it should apply to your reservation. I don't believe they are "attached" to your reservation once your reservation is confirmed.

At Thrifty franchises (which I have the most experience with), the computer systems are too dumb to factor any specific rate rules in. The Thrifty reservation system simply transmits the reservation to the franchise's computer system with a daily rate and a weekly rate attached. Any changes to the pick-up and drop-off dates can't change the rate, because only one rate (with no particular rules) is associated with that reservation.

However, Thrifty corporate locations (which IAD and DCA are) use a more sophisticated computer system that very well could consider specific rate rules such as those noted above. There is a chance--a small chance, but a chance nonetheless--that upon returning the car early, your rate may jump up to the higher rate. I do not think the chance is that big, though, and would feel comfortable booking the later return date and counting on that price to stick.

Worst comes to worst, ask for a manager at return and ask how your rate triples when you're doing them the favor and letting them re-rent the car to someone else for an extra day or two. On an early return, I would (begrudgingly) accept having to pay the same price as I would have had I kept the car longer, but certainly not any more than that. I think the local manager would likely see the logic and agree. Even if you're charged the full $113 for the 8-day rental despite returning it after 7 days, you'll be money ahead. In fact, even if you have to keep the car, go park it offsite, and then call them on the 8th day to have the car picked up by them and they charge you a $25 (or even higher) fee to do so, you'll still be money ahead.