Weddingbee boards

I bought and invitation kit from JoAnn Fabrics because I won't be needing a whole lot of invitations and didn't see any sense in paying a ton for what I could do myself. The thing is the kit doesn't have inner envelopes and I want them so I can clearly mark exactly who is invited (as the number of guests is already at its maximum). The problem is I have absolutely no clue what size envelopes to order for the size invitation I have! The invitations are 5.5 x 8.5 in and the envelopes that are included are 8.75 x 5.75 in. I couldn't even begin to know which size envelope to put it in. Anyone know what I can do for that?