I'm creating a "Total Spent" field that should equal to cost of all past purchases combined. Score is the only field type I can think for adding numeric values to a field. Could I use the score field type and trigger to change the field value when a product purchase is added to the person record? Also, can I use a token for the change value field, inputting the most recent product purchase amount?
Yes, a Score field is simply an Integer field with some built-in functions like add and subtract. (You can add and subtract from Integer fields as well, of course, but not right in a flow. You have to use a webhook.)
And you can increment by a Score Token.