There are two basic meanings in English of the word "PAY."
One is a free market sort of concept where people sees what they need or want, and voluntarily give someone the money for the product or service.
The other is a more sinister concept, more like retribution from someone who has the power to 'make you pay." It's what the Mafia are said to do to knee caps.
So when Trump says the Mexicans will pay, one way or the other, it seems like demanding protection money. Trump said he'd build a wall and get the Mexicans to pay. Now he's trying to keep his promise to his angrier supporters, by demanding Mexico pay 'voluntarily' or, if not, he'll find some other way to punish them.
I do have to say here, that this is not new behavior by Americans toward foreign countries that didn't do as the US wanted. The embargo of Cuba is an example. But there have been many cases of clandestine retribution. The difference today is that Trump says these things out loud.