You're looking for a word for a small thing that has the effect of convincing someone, changing their mind, or concluding a deal, specifically that causes something good to happen.
The clincher: Merriam-Webster gives as a definition "a decisive fact, argument, act, or remark". Their example is negative ("the expense was the clincher that persuaded us to give up the enterprise") but it can also be used of positive things that make you decide to do something. Collins has the example "DNA fingerprinting has proved the clincher in this investigation." You could say "Her doing XXX was the clincher."
A near-synonym of this is the deciding factor, which has a similar meaning, but maybe refers more to making a decision based on analyzing reasons, and less on situations where some minor factor has disproportionate effect. Merriam-Webster defines it as "something that causes someone to make a particular decision".
You can also talk of something sealing the deal. This means literally something that causes someone to accept a deal - Farlex has "I wasn't convinced at first, but it sealed the deal when he offered to include a 10-year warranty for free." But it is also used metaphorically for something that isn't really a deal, maybe something like "She had many good qualities but when she did (something) that really sealed the deal."