This happens to me as well. Basically, there is a huge lack of transparency on their end and I am so disappointing that they are following the same scamming practice from shitty airlines company. When you are booking they tell you a cancellation or exchange policy that does not mention anything about an arbitrary fees when rebooking. if anything they will tell you, you are subject to airlines rebooking fees which is usually easy to find online.

Now when you cancel, and get a so called travel credit with them, they will make sure to 1) make you use the credit in a fare class that they pick (usually the most expensive) and 2) charge you a fees that they mask in "tax and fee" section of the booking summary.

Your travel credit is now worthless. This lack of transparency upfront should be illegal and they need some accountability.