I fly IAH-SFO-SIN-SGN reasonably often. UA to SIN then SQ. Usually I am in business, but this trip SQ will be economy.
Sometime before COVID I had an economy connection in Singapore & was asked to pay $400 in baggage fees (I always travel with 3 heavies).
The agent eventually figured out how to get it done in Shares.

I forget the trick and am wondering if anyone else has had this problem & what is the solution (other than buying a business class ticket).

Thank you