I'm not surprised by that; based on some data points here and from Reddit, the congressional request seems like a super-expedite, where it goes to the top of a special pile and often goes out within a day or two. Seems not unreasonable to me that if you're jumping even the regular expedite timeline then you have to pay at least the same fees that they do Also it seems that some congressional offices will only reach out on your behalf if your travel is within a certain time frame (say, a week or two), and in some cases will ask for proof of travel.

You could probably have the best of both worlds by waiting until a week or two before your trip and contacting your congressperson again; more likely than not it will be done in the regular timeline by then, but if not, you could pay to expedite at that point and still get it in time.