Buy any 2 products (varied prices), set order total to specific amount (not discounting)

[Victoria_CC]

Hi,
I'd like to have an offer that enables customers to select any two products from a collection, or a set of collections, which have varied prices, and for the order total to be a specific amount regardless of the individual products chosen. Not a bundle offer on the product pages themselves but an automatic order total change to *this* amount, not a discount either. Hope this makes sense, any help would be greatly appreciated!

Thanks!

[Victoria_CC]

For example: Customer sees offer "any 2 for $100", they select 2 products from the catalog, go to order and the total amount automatically becomes $100, without showing a discount or percentage off number. Any help would be amazing, if anyone knows of an app to do this I'd also love to know! Thanks in advance!