Am I the only one perplexed at why you should be hiding this from your boss? While they may have a policy to only pay for economy flights, it would be a rather sadistic boss to force employees to only fly economy even if they pay for the ticket personally!

(I'm assuming the OP has to buy then get reimbursed because of the receipt question as opposed to the company buying the ticket directly - where I can imagine problems with the system showing an employee traveling in business against company policy)