I made a change to the tax variable to apply a rate rather than an amount. At this time PayPal doesn't charge tax including shipping as services are typically not taxable.
If this is a requirement I'd recommend calculating the total tax amount on your site before sending the Post to PayPal. The value can be entered in the tax_1 field. I've shown an example of this in the second post.
https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_cart
&[email protected]
&button_subtype=services
&upload=1
&no_note=1
¤cy_code=USD
&rm=1
&item_name_1=Product1
&amount_1=200.00
&quantity_1=2
&item_name_2=Product2
&amount_2=200.00
&quantity_2=2
&shipping_2=6
&tax_rate_1=10
&tax_rate_2=10
&return=http://staging.xxxx.com/store/success.aspx
&cancel_return=http://staging.xxxx.com/store/failed.aspx
Here's an example with you calculating the amount. Notice you're only sending one tax amount rather than a tax amount for each item. PayPal doesn't show itemized tax amounts to the customer so there is no impact on the customer's end.
https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_cart
&[email protected]
&button_subtype=services
&upload=1
&no_note=1
¤cy_code=USD
&rm=1
&item_name_1=Product1
&amount_1=200.00
&quantity_1=2
&item_name_2=Product2
&amount_2=200.00
&quantity_2=2
&shipping_2=6
&tax_1=10
&return=http://staging.xxxx.com/store/success.aspx
&cancel_return=http://staging.xxxx.com/store/failed.aspx