"We are going for five days and one of the credits to use on the last day. Can I pay for one table service instead of using the dining plan? Do I just tell the server that I'm paying for the meal?"
Via Napoli would be a good choice for the Disney Dining Plan - Photo by Cliff Wang
Yes, you can use your Disney dining plan credits at anytime during your trip. Whether it's the first day or the last day. You can pay for meals, even though you are on the dining plan you can pay for meals.
When we took the kids and their spouses back in 2014 (wow how time flies), we all got the Disney Dining Plan for free. We stayed 6 nights so we had 6 table service credits. We planned 2 meals at Signature restaurants (2 credit restaurants) and used the dining plan on those nights and paid cash for the other meals. It was Simple!
You can supplement you're dining plan with cash. In fact, you can pay for all your meals and not use your dining plan at all if you want. Disney doesn't care. In fact, Disney probably prefer you do that. Just kidding, it's a joke, but you can supplement anything with cash. When you go sit at your table, your server is going to ask you if you're on the dining plan, you just say no and then you pay with cash.
I know you're on the dining plan, but you tell them no, because you're not paying with the dining plan, so, and then the next time when you go to pay with the dining pan you say yes.
Dad's Bottom Line
The bottom line is there are no restrictions about how you use the dining plan and cash during your trip. Use it back and forth, it really doesn't matter.