What would you call a block of 10x15 minutes prepaid work?

I sell freelance webmaster services, e.g. Wordpress updates, SEO, text edits and similar to small businesses in my community.

Apart from billing by the hour or by project, I would like to offer a sort of 'carnet', where a client buys 10 "credits".

Each credit could be worth 15 minutes, and the carnet would be valid for a specific amount of time, maybe six months. Whenever they have a small change for their site or I update their site, it would cost them a ticket or two depending on the time spent.

The 'carnets' would be prepaid and non-refundable.

I would also consider including 2-4 credits per month for the clients who host their site with me. These credits would expire each month if not used, and could be used for small updates like change in opening hours etc.

Now to my question. I call them 'credits' and 'carnets' above, but that's mainly in lack of a better word. What would you call them? Are 'credits' and 'carnets' understandable - or should I find another way to describe it?