I have never tried to email them, so I'm no help with that.
But I have been using GWG for four years now -- level 1-5 between my three DC. I would recommend looking at the table of contents for each book on their website and choose a level close to grade level, but that introduces at least one or two new topics for your child. My oldest used the grade 5 book last year and it was almost all review of level 4, just a few new things. She's using something different this year. My DD in second grade is using level 2, but I plan to jump her up to level 4 next year b/c the amount of review will really bother in level 3. But my son in 4th grade using level 4, will likely go on to level 5 nextbyear b/c that type of review is good for him.
Hope that helps.