Credit Card Charges

I have a supplier who we purchase from that sends us an invoice amount and charges the card more than what the invoice is written for. They say it's a credit card charge but it doesn't show on the invoice. example the printed invoice is $169.50 and the card is charged $172.84.  Is this legal?  We are paying out more than we have invoices for! Are they pocketing the extra money since it doesn't show on the invoices?  They are doing this to several hundred customers. Thanks Mike Villani