"Subtotal" vs "total"
It would be advisable to fix it, if the sum is the absolute total, the absolute end sum. Anything before the absolute end sum, such as before the taxes are calculated, or the discounts are calculated, is the subtotal. The amount actually paid is the total, not the subtotal.
If the shopping cart insists on calling the final 'subtotal', he probably has confused the meaning of 'subtotal'.