Authorize.net SIM Payment Setup for my Virtuemart

For some reason, when I create an order an my clients website: [login to view URL], the total does not match up with the total on the [login to view URL] SIM payment form. Experience in Virtuemart is a must. To try out the problem, try ordering food, go to checkout, and click "Pay Using Credit Card/Debit Card". You'll be taken to the [login to view URL] payment form and notice that the total on the website doesn't match to the [login to view URL] payment form total, (for example if you buy 3 Bean Curds with Minced Garlic under Cold Appetizers, it will come out to $19.65, but on the [login to view URL] payment form, it says its $20.25) Need a solution! Exceptional work with be met with future work through my company.