We all know the classical trope of making a deal with the devil. The mortal applicant desires some worldly possession, knowledge, or power, and is willing to trade his own soul for it.
Picture the devils in this setting as the epitome of Lawful Evil. They don't tell outright lies (but if you misunderstand what they said it's your problem), and they always keep their promise (but they interpret everything in a way it benefits them the most). They have a whole army of lawyers at their disposal.
They don't have as their goal to cause suffering in humans, but they are very selfish and do cause suffering if it furthers their goals. Tempting people to commit sins are among their goals, as it will increase their influence and power. However, the people have to do the evil acts willingly, so all the devils can do is to nudge them towards temptations, they cannot force them. They also never arrive uninvited, their PR rules prohibit cold-calling. The mortal must call out to them to conduct any business.
However, they have a problem. The business doesn't go very well. Too few people are willing to relinquish dominion over their immortal souls for all eternity after their deaths. The price is simply too high for any material gain. To expand the business horizons, the commission of the Infernal Corporation decides to offer their services in more limited packages to appeal to a wider range of customers, with lower prices.
However, what can the prices of the limited services be, if souls cannot be split?
Note: the customer will be properly informed that the deal is done with an authorized agent of the Infernal Domains. The services cannot be for free, both because of the nature of the devils and because the customers would become suspicious. The deal has to be fair, or at least perceived to be fair by the customer.
One idea was to do it as a marketing campaign to increase brand awareness, but people who wouldn't give up their eternal soul, would probably not be convinced to do so after trying out a limited offer. And even if they did, giving something for free would be opposite of the very nature of devils. They are selfish, and they only make a deal if it directly benefits them.
Another idea was used in the Order of the Stick, where a wizard got a significant boost of magical powers for a limited time, and as payment, the wizard had to agree to spend the same amount of time as the duration of the powers, in the custody of the service provider at a later time chosen by the provider. In this case the deal was well worth for the fiends because the wizard was taking part in highly important missions, many of them about preventing evil acts, so a temporary imprisonment could make it impossible to prevent one such evil act. However, in our case, the expected customer base is to be regular humans, without such important missions, so the devils would not gain anything really useful out of the deal.
Note, that the service provider has to gain something tangible and useful out of a deal. Just making the supplicant suffer is in itself not a gain. So no such things as "I will help you in your job interview if you hit your little finger with a hammer".
Humans have free will, and the devils cannot directly force anyone to do anything they wouldn't want to. So "you agree that you will kick a puppy" is not an appropriate payment, as it is not enforceable. (and also not too useful for the service provider)
Currently I'm considering "relinquishing control of your physical body to us for an agreed-upon period of time" as a method of payment. It has the advantage of being less severe than giving up one's immortal soul, and it can also be quantified (more time for larger services, less time for smaller ones), and it can also be very useful for the Lowerarchy. However, it has the disadvantage of still being too scary for most potential customers. Can this payment method be improved upon, or are there better payment methods not yet considered?