You can customize your PayPal Payments Standard shipping rates based on currency, region, shipping method, and delivery time. Or you can base your shipping rates on fixed amounts or fixed percentages.

Here's how to calculate shipping costs for purchases made on your website:

1. Go to Account Settings.
2. Click Shipping under "Products and Services" on the left menu.
3. Click Update next to "Shipping Calculations."
4. Click Start under "Set Up Domestic Shipping Methods" or "Set Up International Shipping Methods." If you've already set up shipping methods, you can click Add new shipping method or Add another shipping method.
5. Select the currency and regions, and then click Continue.
6. Specify your preferences for shipping method, delivery time, and shipping rate, and then click Continue.
7. Review your selections, and then click Save Shipping Methods.

To enable your shipping calculations, you must click Use settings saved in your PayPal profile within each button you have created. To offer free shipping, select "Use specific amount," and enter 0.00 as the amount. The HTML code in your shopping cart buttons under "Shipping" overrides the shipping methods in your Profile.

If you calculate costs by weight, you must enter the weight of each item within each "Buy Now" or "Add to Cart" button you create.

If you need more specific shipping calculation options, go to PayPal Developer Portal or contact your developer.