what does OTA contract say about refunds/cancellations?

it's not uncommon/unheard-of for them to charge a handling fee.

if fee is too high in relation to the ticket and above a token value (eg $50 fee for $60 flight isn't that high), you can try complaining to DOT under 49 US Code 41712

https://www.law.cornell.edu/uscode/text/49/41712

(41712 does regulate OTAs)

good luck though. 10-15% is not high, unless you can point to another place on the OTA contract where they put a smaller number



do note that once you request/agree to a refund, it must be processed within 7/20 business days. this includes OTAs. many airlines/OTAs have been dinged for not following this (no, you don't get the penalty, it goes into DOT)
https://www.transportation.gov/indiv...ection/refunds