A price tag, in an of itself, is normally not an offer. There are good reasons for this. For example 100 people may look at a price tag. If it were to be treated as an offer to the world then 100 people could accept that offer and the shop would be contractually obliged to supply 100 of the items when, perhaps, it only has 10 in stock. To avoid this absurdity a price tag is not an offer.
This does not necessarily mean that the price tag is irrelevant. If the shop and customer agree a sale, and no other price is mentioned before agreement, then the contractually binding sale agreement will incorporate the price on the tag.
Exactly when a contract comes into existence depends on the exact sequence of events. It could be just before payment is made or it might be a little while before payment is made. For example if you go into the shop and say "can I have one of those, please" you are making an offer. And when the shopkeeper says "certainly sir" picks one up, and wraps it up for you, at that point there is acceptance and so a binding contract to buy the goods at the price stated on the price tag comes into existence. This is before the shopkeeper says "and how would you like to pay".
On the other hand if, without saying anything, you pick up the goods, place them on the counter together with the exact amount of money in cash, then the contract comes into existence when the shopkeeper indicates acceptance by picking up the money and saying "thank you". So in this particular case acceptance and payment happen at virtually the same time.
Legal SE cannot offer legal advice so I am not saying what the result in your particular circumstances is (more information about exactly what was said and done and when would be needed in any event) but you can see that a key point is whether a binding contract of sale had already come into existence before the salesman queried the price.