Talk to the longarmer - if you're mailing the quilt top and backing, you may want to just buy your batting from the longarmer rather than deal with shipping costs. If you do decide to send it, make sure she can work with the type of batting you have. Some battings that work well for hand quilting, for instance, just aren't good on a longarm. Also make sure she's okay with using a sheet for a backing. Some longarmers don't like to use them due to tension issues; others say they're fine if they've been washed three or four times.
Good luck!