If you have a live rat the snake wont eat, that you cant return, you can put the rat in a covered plastic tub of some kind and put some dry ice in there with it. It will kill the rat, and you can put it in a baggie in the freezer for later use.
My snake had a few issues eating and I tried this and havent had a problem since... ever. Im sure some will say this is bad and will stress out the snake, but it worked for me.
I buy frozen thawed (never more than 3 weeks old). My snake was getting a bit picky and went a couple months without eating. She'd check the rat out but wouldnt eat.
One day, I hung the rat in front of him, and began tapping my snake "lightly" on the nose with the rat. After a few taps, she lashed out and snabbed the rat. The following week, the same. Was curious but wouldnt eat, lightly tapped her on the nose with the rat and she quickly lashed out and grabbed him.
I only had to do this twice and ever since, my snake will always grab what I put in there for her - usually very quickly. I never had to dangle a rat in front of her for more than a few seconds.
Good luck.