Field Name	Description	Required?	Length Limit
Main Fields
cmd	This should always be set to '_pay_simple'.	Yes	N/A
reset	This tells the server to delete any cached button data for the user. Set the value to 1	Yes	1
merchant	Your merchant ID (you can find this on the My Account page).	Yes	N/A
Item/Payment Description Fields
currency	The currency of the payment (BTC, LTC, USD, EUR, etc.). Values supported: Any crypto or fiat currency on the Supported Coins page. Note we in no way process any fiat currencies, they are simply convenience values so you don't have to convert to BTC or another coin yourself.	Yes	3
amountf	The amount (in the currency chosen) of the payment.	Yes	N/A
item_name	The name of the item being purchased.	Yes	128
item_desc	Description of the item being purchased.	No	128
item_number	This is a passthru variable for your own use. [not visible to buyer]	No	128
invoice	This is a passthru variable for your own use. [not visible to buyer]	No	128
custom	This is a 2nd passthru variable for your own use. [not visible to buyer]	No	128
allow_currencies	Currency codes separated with a comma of coins you will accept. This is used to further restrict the coin selection from your list of enabled coins; for example if you are doing your own exchange rates and want to limit checkout to a user selected currency. Example: BTC,LTC	No	255
on1	1st option name. This lets you pass through a buyer option like size or color.	No (unless ov1 set)	128
ov1	1st option value. This would be the buyer's selection such as small, large, red, white.	No	128
on2	2nd option name. This lets you pass through a buyer option like size or color.	No (unless ov2 set)	128
ov2	2nd option value. This would be the buyer's selection such as small, large, red, white.	No	128
Shipping Fields
want_shipping	1 = Want buyer's shipping information. 0 = Don't want buyer's shipping information. (default)	No	1
shippingf	Cost of shipping the item.	No	N/A
taxf	Any tax to charge.	No	N/A
allow_ship_countries	ISO 3166 2 digit country codes separated with a comma of countries you will ship to. The will deny transactions if the shipping address is not in one of these countries. Example: US,CA,GB	No	255
Miscellaneous Fields
ipn_url	Sets an IPN URL. If not set or blank defaults to the IPN URL in your settings.	No	255
success_url	Sets a URL to go to if the buyer does complete checkout.	No	255
cancel_url	Sets a URL to go to if the buyer decides to not complete checkout.	No	255
lang	Automatically set the checkout language to this language code.	No	16
Buyer Information These fields can be used to pre-populate forms with any information you already know about your buyer.
first_name	Buyer's first name.	No	32
last_name	Buyer's last name.	No	32
email	Buyer's email address.	No	128
address1	Street / address line 1	No	128
address2	Street / address line 2	No	128
city	City	No	64
state	State / Province	No	64
zip	Zip / Postal Code	No	32
country	Country of Residence This uses 2 digit ISO 3166 country codes.	No	2
phone	Phone Number	No	32