I hope you know what you're getting into. I've seen this done over and over, with the engineer going along thinking everything's fine and then getting zapped because the drawings/design didn't cover local conditions.
If the designs are submitted for permitting, then they will likely have to be signed and sealed. If so, that should have a minimum price (Say $500). If not, then charge a flat rate per shed (say $350). Make sure your drawings and your contract with the company have plenty of caveats regarding the use of the plans, including review by local authority.