The First Data API page does not provide an API for Ruby but there is a gem called activemerchant that supports First Data/Linkpoint. Their documentation does not mention First Data but I eventually realized that Linkpoint is the same thing.  :-)

Free Resources

• http://railscasts.com/episodes/144-active-merchant-basics
• http://railscasts.com/episodes/145-integrating-active-merchant
• http://www.activemerchant.org/

Random notes

• sudo gem install activemerchant

purchase authorized and executed in 1 ste[

• response = gateway,purchase(1000, credit_card)

If shipping and want to charge later do

• authorize
  • response = gateway,authorize(1000, credit_card)
• charge
  • gateway.capture(1000, response.authorization)

See also gateway.credit and gateway.void

FirstData/Linkpoint specifics

You need to setup a test account with FirstData

• http://www.firstdata.com/product_solutions/ecommerce/global_gateway/virtual_terminal.htm
  • select setup test account
• login to your account and select support -> download center
• get your pem file
• under admin -> fraud setting -> set duplicate timeout to 0 so your tests work

development.rb snippet

config.after_initialize do
  ActiveMerchant::Billing::LinkpointGateway.pem_file  = File.read(File.dirname(__FILE__) + '/mycert.pem')
  ActiveMerchant::Billing::Base.mode = :test
  ::GATEWAY = ActiveMerchant::Billing::LinkpointGateway.new(
    :login => "YOUR STORE NAME"  
  )
end