What's the term for elastic pricing? What I've called "elastic pricing", has a true economics name that I believe I once heard.

Here's the notion:
I walked into a gas station to buy a fountain drink. I only brought a dollar with me, but when I went to purchase the drink, I found out it was actually \$1.89. The cashier accepted that as payment - making a real-time decision that he was able to capture my demand at only $1. I ended up trying this at a few other gas stations, sometimes bringing 90 cents, and it always works.

An alternative to this is that my University just started pricing football tickets at a variable rate based on the location of the seats. Invariably, they were trying to capture the additional value that the second market was instead capturing.

A second alternative is that a former company of mine negotiated every price. So we would usually price the product at $500/user/month. But, we would always assess the situation, and with negotiation offer discounts, or increase the pricing. Or, provide value added services for free or at an increased rate.

So how do I talk about this flexible pricing concept – where a business sets it's pricing based on some kind of average demand, but considers an additional spread based on an individual's demand?

Edit:
In my first example, the bargaining was only noteworthy because it informed a new mental model of mine – that the pricing was flexible. I don't believe it's an inherent component in the term I'm seeking.

As yet another example, consumer package good brands are known to alter their products slightly for online marketplaces – so that value savvy customers can't compare products directly online with those in the store. E.g. Duracell might offer a 6pk of batteries on Amazon vs. a 4pk at a retail outline. Or, they might package the item differently so it has a different SKU.
Perhaps you may want to look into Price Discrimination? This however is the concept of companies varying their prices on different consumer markets. 

1st degree price discrimination, at it's most optimal, is when companies charges the price of each individual consumer's demand. Those willing to pay the most will pay the most. Your description resembles this procedure, but it is not quite exact. Yours implied bargaining, which may or may not lead to 1st degree price discrimination. 

2nd degree price discrimination is setting variable prices depending on the product or the quantity sold, playing off of the consumer's preferences. 

3rd degree price discrimination is the segmenting of a market into smaller markets (based on some characteristic), and charging a uniform price in that market. The price between these segmented markets vary.