When attempting to complete an order, if you receive the error message "Credit limit exceeded", this indicates that your account does not have sufficient funds to complete the transaction.
The shopping cart interface will provide an option to "Add Funds" if there are not enough funds in your account.
Also, if you have multiple items in your shopping cart, it may be necessary to unselect items that you don't want to order at this time as all items are checked off for purchase by default.