A normal sized greenhouse hardly weighs anything. Digging footings etc seems like a complete ball ache and overkill.
Personally I'd flatten the ground. Paving slab it then build off that. You can even get paving concrete in bags. Slap it on the ground nice and flat, put the slabs on it, then either water with a hose or wait for it to rain. It's basically postcrete for slabs.

However if you absolutely positively must have foundations then the standard mix will be more than enough. The standard mix is 1 2 3, 1 cement to 2 sand to 3 aggregate(gravel etc).
Hell, for something like a greenhouse you could probably get away with 1 cement to, what, 4 sand then add aggregate to suit.

Will postcrete work? For a standard sized greenhouse then, yeah, within reason.
Bit bodgy, but it's only a greenhouse.
It's not designed to be laid in long lines so it may crack.
If I had to use postcrete I would get a shit ton of aggregate, some sharp sand, and add it to the postcrete mix to add strength, flexibility, and try to mitigate cracks.

However it would probably be cheaper to buy a wheelbarrow and rent a cement mixer for the day and mixing up normal concrete, then barrow it through the house.

Alternatively you could dig holes and postcrete. Then build 'stilts' off of the postcrete. Then build off of the stilts, which, admittedly, will take longer but should be cheaper.