Sometimes customers have private test suites for purchased packages. How much more valuable would those tests be to both customer and supplier if they were inserted directly into the supplier's development process. The supplier would send out daily email showing the current test scores on the customers tests. For critical packages, this would give the customer accurate early warning of the quality of the next release. The supplier would get yet another set of eyes on testing, and early feedback on that chapter instead of having to play the CustomerStoneGame?.
The customer and supplier could split the value of this relationship by writing a rider onto the usual support contract giving the customer the right (and some means) to submit tests directly to engineering.