I have the \"normal\" PayPal donation button. I want to add two multiple choice fields: Amount and Currency as the image below. So the user
Paypal has specific HTML data requirements when you post to their page. The 'host_button_id' can only be used for one specific value and it's data that paypal holds on their site as a convenience to their clients.
In order to pass variable data you have to pass all the information they need. The 'host_button_id' is no longer needed.
You can find more currency codes here: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_currency_codes
and more html info here: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_buynow_buttons#id08A2G0920QN
Also for the hidden 'cmd' I'm not sure if that will have to change to '_xclick' only. You'll have to test it out and see what works.