are you really opened until 8:00pm and do you cash post office money orders

Yes

Is 1200.00 too much for them to cash?

This is a proxy office. Meaning that they offer the same services as your normal post office but rely on the main office to come by and pickup their mail. They are located inside a deli thus follow the unusual hours. Since you're not mailing something but dealing with money order I would assume yes. However, if that's a large sum of cash they might not have it in their register and you might need to visit the mail office.