We are doing invitations by mail and email

May I ask why both? It seems unnecessarily confusing (you may have people who RSVP twice - or not at all - because they received two invitations).

Honestly, I would recommend mail only. E-mails get lost/moved down the inbox/sent to spam/etc. so you'll probably have to chase more people down (plus if you're inviting any older people, things may get really interesting).

You can get inexpensive invitations from zazzle, shutterfly, minted, etc. if cost is the issue and have people RSVP online.

I know how crazy all of the planning gets (and especially toward the end when you're trying to make sure you have everything together) and you don't want a reasonably straightforward process like invitations and RSVPs to make your life more complicated.

We are doing something like this, we have guest coming from different countries and mail often gets lost or takes forever to arrive, so we have decided to mail a "save the date" card, a long with the wedding website+QR code to make it more accesible.

The website would contain all the information related to venue, hotels where to stay, addresses..etc..

it's the most convenient thing or us

If you have a lot of guests, then it seems to me that email invitations are the best option