While bp.tf does show a price on the grenade, it is not accurate in this case, as it is probably showing the price from a previous SCM seller that is not selling anymore. The last SCM sale was in November of 2020:

https://steamcommunity.com/market/pricehistory/?country=US&currency=1&appid=440&market_hash_name=Professional%20Killstreak%20Rainbow%20Grenade%20Launcher%20(Factory%20New)

With that said, it means you need a mini for it. What you paid for it would be great as a mini, however I cannot trace your purchase through compares. Can you get a screenshot of your purchase and use that as a mini?