Payment with multicart can be done with any of the following methods.

Authorize.net payment

Provide your credit card number,validation code,expiry date

Your Pay payment

Provide your credit card number,validation code,expiry date

Google Checkout Payment

Select this payment gateway you will be redirected to google checkout payment server.
After you made the payment you have to contact the administrator to confirm your order

Gift Coupon Payment

Enter the gift coupon code received by selecting this payment method.

Paypal payment

When you select PayPal as your payment option, you will then be directed to the PayPal website. Here, provide your PayPal log in information. Once you confirm the payment in PayPal, don't forget to select 'Return to Merchant' to finish your order.
After you pay for the order, it will be placed in the multicart. To manage your order go to Manage Orders