How do you decline going to a wedding after the couple sent you this? Keyshawn Johnson just sent out the BEST wedding invitations ever -- iPad 4s -- each pre-programmed with all of the 411 for Key's big day ... TMZ Sports has learned.

Here's what we know ... the iPads were sent to several guests a few weeks ago -- and once they're powered on, a program opens informing the recipients about Key's wedding events.

Also included -- a photo gallery, guestbook, and countdown to the big day.

The couple also made it easy for guests to buy wedding gifts -- by including the registry right on the iPad.

Of course, the woman Keyshawn is set to marry is Jennifer Conrad -- his longtime GF and the mother of 2 of his children. It'll be his second marriage.