Buying Extra Baggage (Delta Tick on VA plane)

Hey all,

I called VA to check if I could book extra baggage for my return trip, but I was told I cannot do this, and would need to check if I can at the airport....

I booked us tickets with Delta, on a codeshare with VA, Gatwick to Vegas/Rtn......

Don't mind paying $100 for extra bag at airport, but just wondering if anyone has done this before, ie: paid for an extra bag at the airport ?
There are no restrictions that state as a Delta ticket holder, I am unable to purchase extra baggage, Delta list off the costs for extra bags anyway....

Just have that slight worry, I get to the airport on way home, and the check-in desk for the Virgin flight will say to us we cant pay to take an extra bag on board as we got Delta tickets, not direct VA tickets ?
Am also guessing I am worrying for nothing and it will be OK, but doesn't hurt to ask !

I have added on extra baggage onto a flight for the return leg only for my DD before. This was on VS metal both legs. I am pretty sure I did it directly online through my future flights option. Not sure why you couldn't do this on a codeshare Delta flight, seems a strange one.