Phrase/word between "bait-and-switch" and "additonal sale"

Solution 1:

I believe this type of thing is called a "loss-leader" - supermarkets use the tactic to draw consumers in on the promise of a few of their product lines being cheap - knowing that in the course of their shopping - customers will probably buy other goods at higher prices. Another description is that of "upselling"which is another tactic used by online retailers who tempt buyers with a small purchase and then - at the point of ordering - try to sell additional items which are usually higher in price.

Solution 2:

You probably mean lock-in, though the initial sale triggering it is not necessarily something small. When a sale causes a lock-in condition, the buyer is forced to buy more from the same entity in future to continue enjoying their initial investment be it through consumables, spares, upgrades or maintenance services.

ODO:

lock-in NOUN

1 An arrangement according to which a person or company is obliged to deal only with a specific company.

‘This creates a lock-in condition for the customer, which means higher costs for upgrades, service and expansion.’
‘The customer base is an important element of market power for aircraft manufacturers since there is at least to some extent a lock-in effect for customers once their initial choice of aircraft is made.’

Solution 3:

Gary Cooper's recommendation of up-selling comes close, but since the seller's goal is to keep you buying more things over a period of time, the more accurate term would be cross-selling.

Up-selling usually refers to getting a customer to purchase a more expensive version of what they came for, or extra accessories. Examples would include the sport/limited edition of a car, a cover and insurance for a tablet computer, or "fries with that."

In cross-selling, salespeople target existing customers and tell them how great this new thing they have (camper trailer, motorcycle, certificate of deposit) goes with the truck, moped, or checking account they already own. (Of course, the customer has to know this is happening, otherwise the salesman might be working for Wells Fargo. [obsnark])