It looks like that is a 15% tolerance amount that you card company imposes until the charge settles.  Your card company must classify us as "mail/internet orders" and do an automatic tolerance hold charges coming through with that classification.

It's kind of like when you purchase gas using a debit card and they will do a tolerance amount anywhere from $1 to $100 or when renting a car, things like this.

 

 

Hope this helps explain:)