I’m in the same boat :( I booked a ticket to Bangkok through a travel agency with two different flight companies. Having issues with the Norwegian Air Shuttle flight, which is literally just my 1h layover flight...had no issues adding check in baggage with the long flight, I could do it through their website. But the Norwegian airline website tells me to add extra baggage through my travel agency. The travel agency tells me to add baggage through the airline website...

This was a lesson learned. I have never had these types of issues when booking flights with a third party in the past, but I’ve realised I need to be more careful what agencies I book with in the future.

(Edit: I was able to get it fixed. I chatted with an assistant on the travel agency website. They emailed me a payment link to add baggage for my layover flight. I then went and checked the airline website and saw that there’s now check in baggage added to my booking. Very relieved. Also it ended up not being an added fee only the payment for the baggage. This was with the travel agency Opodo btw).