Memo to the Birthday Industry

Ok, so I'm planning my daughter's 6th birthday. I go to the store to get printable invitations, and I need about 13 or 14. They come 10 in the box. So I guess I'll have a few left over. Then, I go to get the paper goods. 8 napkins come in a pack, but the plates come in 12. Can you tell me why? The party favors are the same way. Some are 4, 6 or 8 in the pack. The gift bags come in 12, so I had to buy two packs of those. The bubbles came in a 24 pack. I'm going to end up with all sorts of extras from buying different quantities. This is really driving me crazy!