Josh,
The 'invoice_id' field is mapped to ActiveMerchant's :order_id option, the 'description' field is mapped to AM's :description option, and the 'email' field to AM's :email option.
This would look something like the following:
gateway = ActiveMerchant::Billing::AuthorizeNetGateway.new
(:login => 'l', :password => 'p')
response = gateway.purchase(100, credit_card,
:order_id => '1025',
:description => 'Store purchase',
)
# The transaction ID is returned in the method Response#authorization
puts response.authorization
Take a look at the file remote_authorize_net_test.rb for some good examples.