There's lots of ways to compensate a cat! Not all sales compensation plans are commission-based (a percentage of the sale). Commission-based plans are much less common for sales engineers vs. salespeople, unless they are working with only a single salesperson.
In general, sales engineer variable compensation plans are going to be based on some mix of sales numbers, with less variability and risk than the salesperson's plan. Ideally providing goal alignment with money, smoothing out the stresses, but also encouraging some level of independence for the sales engineers. A ward against salespeople who might be seduced by the Dark Side.
The less obvious case you raise, a single engineer directly responsible for causing a customer to spend more money: structurally or ad-hoc, that person should probably be getting paid. Maybe the customer-facing people aren't engaging efficiently, and it's a sign of something a bit out of alignment with customer operations.
Salespeople can provide big value in getting engineering attention to the right customers at the right time... but the best ones know it's a team sport. I know salespeople that give envelopes of cash to difference-makers, though that's exceptional.