POS that handles cash amount transactions?

My general question is, what do gas-station POS machines do differently than other retail/restaurant POS systems in regards to specific-amount cash sales? Example, walking into a Chevron and saying “Give me twenty on pump four”? I haven’t seen a clerk divide requested amount by $/gal and input that into the machine, so I assume the machine has the functionality to do this itself.

My specific question is, can someone tell me if I can get this functionality from a readily available app? Or is this something any POS can do and I’m missing how to do it entirely?

I ask because I’ve been looking for a POS app for Android that will handle keeping records for the small business in which I work. We are a small retailer of biofuels, and being small, we don’t have/cant afford a gas station-style POS machine. Having something that will work on our cheap Android tablet will help immensely, as we currently do all our record keeping by hand on a ledger.

If you ask for $20 the clerk enters $20 for the requested pump. How many gallons that equates to doesn’t enter into the equation. The pump stops pumping when the dollar amount gets to $20.

General note to visitors to this thread: “POS” as used here stands for “point of sale”, not “piece of shit”.

Being a programmer of non-POS systems for a few decades, I really have no direct experience in this area, but I think I can make some educated guesses:

I don’t think the POS at a gas station does much at all different than the one at a restaurant. The one at the restaurant says “Okay, that was a $23.52 sale”, and then it communicates with the bank about. It may know how much pizza and soda went into the $23.52, but it doesn’t really need to. Similarly, the one at the gas station only cares that you bought $20 of gas – it doesn’t care how many gallons, or whether it was regular/premium. The main difference is that after you’ve paid, the one in the restaurant is finished, but the one at the gas station sends a signal to the pump to let the gas flow for a while. But - like Ex_Bubblehead said - whether that signal says “Stop at X gallons” or “Stop at X dollars” is pretty trivial to the electronics.

I have worked with one particular POS system (in both senses of POS :smack:) for about 20 years now, first as an employee of the software company that wrote it, and later is a free-lance consultant.

Our cash register had an important feature, that I’m going to guess ALL POS systems must have to be very useful and salable: It is highly customizable!

Perhaps most programmers today know that Microsoft apps can all be extensively customize by means of Visual Basic scripts that anyone can write, that can be plugged into the Microsoft app as an add-on module. A great many other third-party apps can be enhanced with third-party add-ons likewise. Our cash register did likewise, although with its own home-grown scripting language rather than Visual Basic. I myself have been writing customizations for much of my 20 years with this product, which range from one-line changes to existing scripts to 200-page new scripts written from scratch.

What the OP really should be looking at is this: What apps out there can be customized like this? He may or may not ever find apps that do just exactly whatever he wants, but he also might find apps that can be customized via Visual Basic or whatever scripting language to do what he wants. What you want is an app that is extensively customizable; which has a reasonably well-designed and well-documented “document object model” or something equivalent, and a reasonably well-thought-out API by which your custom scripts can be hooked into various events that happen in the app, to modify their actions. (My POS system basically has none of that, and has required much “creativity” on my part to get a lot of things done, much of which even the programmers who wrote the original app seem never to have imagined.)