Found the 'Better Together' order total contrib:
osCommerce Community Add-Ons

It doesn't have a GUI in admin but it is so versatile that I'm desperate to get it to work.
It will allow you to offer products as 'tag along' at a discount or for free. (not really different from the 2gether contrib) BUT it also allow you to link a complete category to a product, or link two complete categories.
So you can sell a keyboard and offer a discount on your complete category of mice!

Now comes the frustrating part:
The actual 'engine' is an order-total module that just gets dropped into the ./includes/modules/order_total/ folder. Install in admin (modules>order total)
It works on a plain osC install but not in oscMax and I can't figure out why. What happens is that during checkout, parsing of the checkout_payment.php grinds to a halt just below payment method selection. (and thus just before the CCGV box)

I've tried it in the current osCMax 2.0.2 and in an older 1.7, both with the same result.
Because the checkout_payment page fails just before the CCGV box I'm thinking the CCGV might be the culprit.

Does anyone have an idea?
Thanks in advance!

Regards!
Erwin