Offer it as SaaS if those are your considerations. Just bear in mind you'll need to have a licensing agreement written up to cover things such as availability, maintenance, billing.
Alternatively, and I'm just throwing it out there, 70 to 100 hours isn't a large amount of time in the programming sphere. Somewhere between 1 and 2 weeks work and so not particularly valuable vs providing the support and upgrades which you're not keen on (at which point you're specialist freelancing).
Many of those may have a SaaS model you may want to look at to fix that problem. Otherwise I know PDQ Inventory does allow you to get product keys, if I recall.
RE: LicenseBox - PHP License and Updates Manager v1.4.0