The Load manifest is a simple one page, takes you two minutes to fill out, form. It is just one must have it or risk the not-so-friendly folks at Customs getting awkward about it. It must be noted the only place I felt mis-treated was at the land crossing at Sweet Grass, Montana....must be the agents in question were not at their posting of choice from what I saw. Sent an unkind message to their Inspector General's Office about it.
Don't put it off without checking with the FAA and the Customs guys....some awfully pretty flying in the States. Both have stretches of miles of miles of miles as you cross the prairie land. Flat...square....windy. Some awfully pretty corn fed ladies however.