1. They're good customers who always pay on the nail.

2. You have to pay on the nail before you can leave the shop with your goods.

3. Not paying on the nail could be extremely expensive.

4. They always pay on the nail. They are never a day behind.

5. We have to pay on the nail before we can leave with the goods.

6. They always pay on the nail. They're never a day behind.

7. It is my usual practice to pay on the nail when going shopping.