Sometimes PoS machines, or the people operating them, will ask if I want the receipt BEFORE I hand my credit card over for payment. What would be the feasibility of charging more money than the person/machine says they are going to, knowing that I wouldn't be able to dispute the charge without the receipt? Is it something I should be suspicious or wary of?
In my experience, even with such POS machines, there is a display that asks you to confirm your total, as a very last step, even if they ask if you want a receipt as a first step.
By this manner, it would be nearly impossible for the teller to change the total without you noticing, so long as you read the total given to you by the device.
The scam in question is more likely to happen in a restaurant with the additional tip segment, allowing someone to give themselves more of a tip than you initially wrote onto the receipt. This is the only way I believe you would have a feasible chance of being cheated like this.
Since the POS machines are tied into the register it would be rather difficult to overcharge with an attentive patron. They would have to add an additional item onto the purchase in order to increase the total before running the card (very few system allow cashback to be requested from the teller side), and most machines have audible cues every time an item is added. If you are paying attention to the teller and not talking/playing on your phone (or other distracting things) then I would say the feasibility is probably very low.
Except for rare exceptions while traveling I only shop at locations where I can see the total on the register, and make sure it looks correct before handing my card over.