It's a Random Event. I miss them all the time and find the invite in my inventory.
The invite will let you see what's in subeautique and let you buy a single item from there. If you collect wardrobe and want items from there I'd keep it and buy an item I wanted. Otherwise there is always someone to buy the invites.
[flower=jersey]
“Justice will not be served until those who are unaffected are as outraged as those who are.” - Benjamin Franklin
"Woe to those who make unjust laws, to those who issue oppressive decrees, to deprive the poor of their rights and withhold justice from the oppressed of my people, making widows their prey and robbing the orphan." - Isaiah 10:1-2
Also unrelated to that, what's the cap on prices in a store? I assume if an item is worth more it is then sold in the trading post?
The invitations are given out via random event. Whether or not to sell it is up to you; do you want sp from the sale or one time access to Subeautique?
you can usually make a decent profit by redeeming the invite, buying something (i like to check the prices of everything there and buy the most valuable one, or wait until something especially rare/new pops up but those are hard to catch because people are really quick with them) and then selling it.