They will ship it to you first and then you pay later, really sounds like they are desperate to sell one of the most sought after drugs in the World, I'm not an expert but in the UK we have a program when rich kids go homeless and he was with a few crack and heroin addicts and they said when they hear of someone who OD they want to find that dealer as that is the good stuff. There are plenty of YouTube video's as well which may to helpful to make your mind up, I wonder why they are risking selling them on the open market when they can get top $ from any local Heroin dealer.
Do you have to give them your bank account details first I can't see them just sending them hoping you pay them later what payment options do they offer.
Below I hope you find the information your looking for all methods have been discussed, Good luck
https://sanctioned-suicide.net/search/304360/?q=Fentanyl&o=relevance
Cheers
Geo