Why do I get debited twice?

[Angel]

I am located in Europe. Every time I make a purchase, like paying my leased galleries from a US based company, my credit card gets debited twice. Once for authorization then a second time when the charge goes thru (both for the same exact amount). It then takes 7 days before my credit card is credited back for the full amount of the first authorized transaction. This only happens in the US. How come?
I hope I made it clear enough. LOL