BigCommerce Fix - open to bidding

Hi, Im looking to customize my Bigcommerce site. [login to view URL] Its still currently in it's free trial. What I wish to do is for my Invitations to be able to sell matching RSVP and Thank You cards on the same page at different quantities without having to add each one of them as a seperate product. What I have done is created a pick list as an option for the card with a generic 100,150,200,250,300,350 card count of rsvp/thank you cards.. [login to view URL] Now when I purchase 50 Tie the Knot cards at 8,20 each it totals = $410 which is correct Now when I purchase 50 Tie the Knot cards at 8.20 each and 100 RSVP at 90$ total it totals = $4910 when it should total $500 The reason is it is adding my 100 RSVP card cost to each 1 invitation, instead of to the total order. I wish to fix this but was told I would need to create a product matrix for this. What would you charge for this service and how long would the process be? I need a solution that I can apply to all my future products as well.