The sentence 'you may do so' is correct and usable in written English. You can use it when you want to give someone permission to do something or give them the option to do something. For example, you could say: "If you'd like to take a break, you may do so.".
"You may do so now".
If you have not yet read this jaw-droppingly wonderful Craigslist "Missed Connection" posting, you may do so now.
And so even if you've renewed that gift subscription many times, you may do so again at the less expensive rate.
If for some reason you failed to take advantage of that no-longer-exclusive but still exciting offer, you may do so now.
If you are buying a frequent-flier ticket using points you've accumulated through an airline rewards program and you must cancel your flight, you generally may do so.
But, some may do so if you request it.[7].
It may do so again.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union