If you are not eligible to contribute to an HSA in 2018, then it doesn't really matter when you pay the bill. The deduction for medical costs has such a high threshold (and may go away) that it probably won't affect your taxes one way or the other.
However, there is one trick you can take advantage of. Don't close the HSA, leave it open with a few dollars in it (whatever the bank minimum is to keep the account open and not get killed with fees.)
Because, you can reimburse yourself in the future for medical costs you pay out of pocket now if the medical costs occurred after the HSA was opened. So let's suppose that you change your insurance in 2019 and are again eligible to contribute to an HSA. If you open a new HSA, you can only cover expenses that occur after that HSA was opened. But if you leave the current account open, you can deposit money into the account in 2019, then take it right back out to reimburse yourself for your 2017 expense that was paid with out of pocket funds (whether you paid it in 2017 or 2018). Essentially you can "wash" $2400 through the account and claim a tax deduction for it and then use it to reimburse yourself for the old expense--if you keep the old account open.
