Flying VA on a Continental Codeshare

My son is moving to Nassau on Monday and his employers have arranged a Continental ticket, on a VA aircraft via MIA. He needs to take excess baggage, but the cost is different on each web site - 75 with VA and $100 with Continental.
Will the VA excess charges apply or the Continental? And as his onward flight is with AA, will he need to pay more again for the extra bags. He has a through ticket.
Thanks
Will the VA excess charges apply or the Continental? And as his onward flight is with AA, will he need to pay more again for the extra bags. He has a through ticket.
Thanks