Field Name	Description	Required?	Length Limit
Main Fields
cmd	This should always be set to '_pos'.	Yes	N/A
reset	This tells the server to delete any cached 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_name	A description of the checkout's purpose (ie. Checkout, Sale, Food, etc.)	Yes	128
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	Preload amount (in the currency chosen) of the payment.	No (unless allow_amount = 0)	N/A
Optional/Informational Fields
allow_amount	1 = Allow buyer to adjust item price. (default) 0 = Don't allow buyer to adjust item price.	No (unless allow_amount = 0)	1
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_currency	0 = Don't allow buyer to adjust source currency (default). 1 = Allow buyer to adjust source currency.	No	255
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,LTCr]	No	255
ipn_url	Sets an IPN URL. If not set or blank defaults to the IPN URL in your settings.	No	255
lang	Automatically set the checkout language to this language code.	No	16
cstyle	Sets a coin selection style for the checkout page. (default: grid1) Warning: Other styles may not look right, they are untested	No	32
Buyer Information These fields can be used to pre-populate forms with any information you already know about your buyer.
email	Buyer's email address.	No	128