Also, will credits last forever? What I mean by this is that when you buy some credits, they will never expire until they are used for rendering?

I think it's quite clear on the credits page?

Resolution, max 1million pixel, for example 1350x740(999900 pixel).
Samples, max 250.
Number of Parts, max 1000.
Formats, png & jpeg.
No special features like compositing, depth of field, wear and tear, discolouration, flatness distortion and parts randomisation.

No they don't expire. Credits are subtracted from your account when the project is successfully rendered not before.

Thanks for the info. You just might have another customer! 😉