Student or Learner
To start with attached your credit card and an exclusive offer booklet
your credit card and an exclusive offer booklet is attached herewith
Instead of "attached" use "enclosed" and I would delete the "herewith."
Your new credit card and exclusive offer booklet [that lets you... do what? take advantage of reduced prices at X fabulous restaurants, hotels, and shops?] are enclosed.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.