You might be analyzing the sentence incorrectly. This is understandable. According to the standard rules of punctuation, Amazon should have added hyphens to assist you. If they had done so, the text would look like this:
You can find all your off-Amazon account activity here. Click Amazon Orders below to see your on-Amazon account activity.
As you can see, off-Amazon and on-Amazon are being used as adjectives to modify account activity. The hyphens make this clear.
The Chicago Manual of Style describes these structures as phrasal adjectives:
A phrasal adjective (also called a compound modifier) is a phrase that functions as a unit to modify a noun. . . . Generally, if placed before a noun, the phrase should be hyphenated to avoid misdirecting the reader [emphasis added].
Yours does not fit the context because it is a possessive pronoun and nothing is being replaced here. The possessive adjective your does fit the context.