How to express an unwanted purchase?
The words "upsell" and "cross-sell" are both defined below. While upsell is used more frequently, cross-sell may more accurately fit your sample sentence and be applied here.
Upsell - verb
to try to persuade a customer to buy a more expensive item or to buy a related additional product at a discount
They're always trying to upsell you on that jumbo popcorn bucket at the movies.
Cross-sell - verb
- to sell or try to sell (similar or related products or services) to an existing customer
- to suggest that customers buy additional, complementary, or related accessories or products during or just after their primary purchase
http://dictionary.reference.com/browse/upsell
You could say:
I needed to buy only a shirt but they foisted a sweater on me too.
Quoting Oxford Learner’s Dictionary, foist somebody/something on somebody:
to force someone to accept someone or something that they do not want
Here is an example that's similar to yours:
I have, however, owned several cars—some of which were overpriced, underperforming clunkers foisted on me by slick, high-pressure, promise-'em-everything desperados of the dealerships. Rose Mula, The Beautiful People and Other Aggravations, Pelican Publishing Company, 2010, p. 181.
If you prefer a milder alternative you could say:
I needed to buy only a shirt but they managed to sell me a sweater too.
I needed to buy only a shirt but they succeeded in selling me a sweater too.
You could say, they twisted my arm to also buy a sweater or I let myself be arm-twisted into buying a sweater too.
twist someone's arm
To persuade someone to do something that they do not want to do He might help us with the painting if you twist his arm. (humorous) Have a cream cake?' 'Oh, go on then, if you twist my arm.
Cambridge Idioms Dictionary, 2nd ed.
(idiomatic, by extension) To coerce, force, or cajole (emphasis is mine.)
Wiktionary
The woman serving me twisted my arm to also buy the powder, and I sure am glad that she did. BeautyLifeLeah
Alternately, consider:
sweet-talk [someone into doing something]
sweet-talk: To coax, flatter, or cajole (someone)
talk into: v. To persuade to by talking: I talked him into buying the house.
Collins English Dictionary
I needed to buy only a shirt but let myself be sweet-talked into buying a sweater too.
soft-sell (something to someone)
soft′ sell′
A method of advertising or selling that is quietly persuasive, indirect, and sophisticated (opposed to hard sell). [1950–55]
soft′-sell′, v.t. -sold, -sell•ing, adj.
Random House Kernerman Webster's College Dictionary
I needed to buy only a shirt but was soft-sold a sweater too.
fob off/palm off/pawn off (something on someone)
v. To get rid of or dispose of something by fraud or deception.
The American Heritage® Dictionary of Phrasal Verbs
I needed to buy only a shirt but they pawned off a sweater on me too, saying it's the best piece money can buy.
lull (someone into doing something)
lull: to give or lead to feel a false sense of safety.
Random House Kennerman Webster's Dictionary
I needed to buy only a shirt but let myself be lulled into buying a sweater too.