How would you use "can" in a future sentence?

Solution 1:

Will and can are both modal verbs, and in most varieties of English* there is only one slot for modals, so they cannot both occur at the same time. There is some discussion on that issue here: Can I say "We don't must", any alternative using a modal verb if I can't?

As others have pointed out, the future is not precisely necessary here. English embedded clauses tend to take present tense, so the following is probably close enough for your meaning.

If you purchase this then you can do that.

There is some relevant discussion on that matter here:

Why do I instinctively want to use the present tense with a conditional?

*= There are dialects that allow some "double modals," like might could, but as far as I know, this doesn't include will. In any case, it isn't standard.

Solution 2:

Will be able to is the future tense of can, therefore the sentence you presented is as close as you can get to express what you're trying to express.

Solution 3:

If you purchase this then you can do that.

However, I would leave the sentence the way it is, as it reads more formally. "Can" has a very colloquial feel to it. It depends on what you're using it for, of course.