First, it's AE, so "post" doesn't typically mean "mail" (as codero hints).
What's being discussed are "post-it notes" -- those typically yellow small squares of paper with a bit of gentle adhesive on the back so that you can write something on them and then stick them to some surface.
There's been a discussion about registering for gifts here recently, but there's a great deal of wild speculation about what it is, much flat-out wrong: Siehe auch: Geschenketisch
A gift registry is when the bride/groom/couple goes into a store and makes a list of the items from that store that they would like to receive as wedding gifts. Nowadays, that list is stored in a computer so that it can be retrieved from various branches of that store. Usually, the person to be married lets the potential gift givers know where he/she/they are registered (perhaps only if they ask). The gift givers go to that store and look at the list. If there is something on there that they would like to give and they buy it there, the salesperson will enter that it has been purchased (to prevent that the couple received 20 blankets, for example). If they buy it elsewhere, it's up to the receiver to let the store know that the gift has already been received (in the US, it's not uncommon, and in some cases preferred, to give gifts before the wedding -- perhaps individually, perhaps at a bridal shower).
Back to the OP: She's just reminding herself that she needs to complete the gift registry at at least one store. Nothing is being sent to the guests, as has been speculated here incorrectly.
From what I've read here in other posts, such registries exist in some form here in Germany as well.