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

他们是好主顾，付账从不耽搁。

《牛津词典》

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

我们必须当场付款，才能拿走货物。

youdao

Almost all of li's customers - traditionally wary of falling into debt - pay cash on the nail, in full, for their autos.

几乎所有李先生的客户——他们对负债有天然的警惕——都会马上以现金支付全部车款。

youdao

The Baltic states pay cash on the nail for their gas, and offer the Russian oil industry its most efficient export routes.

ECONOMIST: Still most awkward partners