It is quite easy to stumble over the calculation rule system, when you are not used to it. The strength of VirtueMart's calculation system lies in the variety of possible combinations. The most interesting attributes to combine are usually the product categories and the shoppergroups. Consider that you can use unpublished product categories, to layout an hidden structure about your products and to create with it a fine grained calculation. Of course to take advantage of this power your products should use in the price definition the generic/standard setting. If you choose a rule there it is always used, regardless any restricting attribute.
We have two primary types of rules, per product and per bill/cart. Rules per product are calculated for each product. That means you see it in the product details view and per product listed in the cart. The rules per bill/cart are using the subtotal of all products and is only visible in the cart respectivly the cart module. Each calculation rule is assigned to a designated role:
- Profit margin
- price modification before tax
- Value Add Tax
- price modification after tax
Combined with the multi currency system of VirtueMart and the profit margin you can dynamically calculate your desired cost or final price.
The price modifications can be used as discounts or something else. Any role can be used with any mathematical operation.
The tax is meant for goods which are taxed by weight, unit, or similar. The VAT Tax is recalculated and lowered by given discounts.
The other attributes of the rules create dependencies, for shoppergroups, categories, manufactureres, countries, states, and date. As always we follow the simple rule that "no entry" means "all". So just leave them empty at first.
Notes to the rules per bill
Only categories are correctly calculated, it does not work for manufacturers.
Some notes to the vat tax
If you use a "VAT tax per product" for your products, then the VAT tax is lowered if any discount is applyed after. It happens very likely that you have products with different vat tax in your cart. If a discount or coupon is used, then the vat tax is lowered by the correct relation of the used vat taxes.
You can also use unpublished categories to add a tax to your products and set then the correct category in the vat tax rules per bill.
Countries and States need the address of the user
Here some more abstract overview about this topic http://docs.virtuemart.net/manual/general-concepts/204-tax-and-calculation-rules-overview.html