Once you complete the online form you submit it electronically, print it out, schedule your biometrics appointment, print out the confirmation & then pay for it.
You then *after attending biometrics* put everything together (biometrics sheet, printed application, all your documents & passports) and overnight it to the Consulate for processing. If you want to use the Settlement Priority Service you purchase it ON THE DAY just prior to mailing it to the Consulate.
Technically they could refuse you as you wouldn't be 'living together' for the 4 years at the moment you apply for your visa...but I've not seen that happen to US applicants...yet. Considering that they're starting to be stringent about the interpretation of the rules it could happen in the future.