Hello Stefan,
You wrote:
Uuuh, what makes you think there is none?
It's about the support packaging:
Donation-based support is very hard to get manager approval for.
The other options are somewhat hard to explain to someone who's to approve purchasing support, I think. I would suggest looking at how Mathias Kettner's Check_MK support sells packages of "support credits" (which run out after a year or so, so that the support organization doesn't over the years build up a pile of obligations to support credit package holders). The support credit packages may be bought using a credit card form[1]. Then, when there's a need for support, it costs X credits, based on the complexity of the case (which might include feature-development.
Notes: 1: Being able to get electronic invoices would be even better: http://peppol.eu/ http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014L0055