You can, and should submit almost everything electronically. Schools will request your application as soon as you pay the fees and they get notice of it.
However, note that schools will consider your application complete as soon as they have everything they REQUIRE. This means if they consider LOR optional, they may read you application without it.
If you do send an application that is incomplete, drop the school an email asking them to hold you application until you submit whatever materials are still outstanding.