Sandy, if you save your work with Save as… and choose the .pspimage format, it will keep all your layers intact so you can continue your work later, or revisit it if you change your mind.
If you want to merge it, you can simply save it as .jpg and it will automatically merge all the layers into a single one, which you can then post (but you will also have to resize it to 600 pixels and save a copy with -600 in the filename so it won’t overwrite the “good” version).
