Proper usage of 'if so' in a sentence

I have used a sentence today like below but not sure I used if so correctly here;

Additionally, we could make your booking via e-mail if you prefer to proceed that way. Please let us know if so.


I would just move this around slightly:

...we could make your booking via e-mail if you prefer to proceed that way; if so, please let us know.

Or, you could re-word to get the same meaning:

...if you prefer, upon request we could make your booking via e-mail.