In English, this would most likely be called a confirmation code. "Safe" doesn't sound right to me here. I think
"código seguro" means "secure code", and "verificación de la expedición" would be confirmation or proof of issuance -- but putting all of it together unfortunately isn't pretty in English, at least not in a way I can think of...

IF the concept is explained in the pertinent section of the website, I would suggest just using confirmation code; if not, maybe you could rephrase it to say "secure code for proof of issuance" or similar.
HTH