Hi Gordon: Here's what BFG help section says about purchasing Collector's Editions. Looks like you can't combine things, but maybe BFG will have a different solution for you:
"You can use 2 Game Club credits to purchase a Collector's Edition, even if you're no longer a member of the Game Club. Just add a Collector's Edition to your shopping cart, and if you have 2 Game Club credits on your account, they will automatically be applied to the purchase.
Please note that you must have at least 2 credits on your account for this transaction, and that one credit cannot be used to partially discount a Collector's Edition.
What if I only have 1 credit available? Can I pay the difference?
At this time, you must have 2 Game Club credits available in your account to purchase a Collector's Edition. We are not able to charge a partial amount to another method of payment. If you wish, you can purchase additional credits at any time from your My Game Club page and apply them to your next Collector's Edition purchase.
Can I use a Punch Card coupon, gift certificates, or a free game coupon instead of a credit?
At this time, Punch Card coupon, gift certificates, free game coupons, and other discount coupons cannot be applied to the purchase of a Collector's Edition."