It's good to ask, but "may opt" does not equal "have to." In the standard way that contracts and leases (and, for that matter, statutes and regulations) are interpreted:1) Words are given their usual and customary meanings, unless specifically defined otherwise; and the usual and customary meaning of "may opt" is that the person has a choice but is not required to.2) Even more specifically, "may" is consistently interpreted by the courts to be permissive, not mandatory; when someone intends to make something a requirement, instead of "may," the word "shall" (or "must") should be used.So you are not required to pay for or otherwise due the cleaning yourself. What you've done is probably the better choice; from what you rights, it seems that even if you had  opted to clean the place yourself, the landlord might be able to still charge you for the compulsory cleaning as well, if the landlord believes the cleaning you paid for or did was inadequate.