Hello Emily,
The right conjugation for "eu" is "Eu comprei..."
"Comprou" is the conjugation for "ele", "ela" or "você" in Portuguese's equivalent to the simple past. The correct conjugation for the second phase would be "Eu comprei um perfume por cinquenta reais." and the first phrase is right ("Eu comprei tanta coisa".
Is interesting to search information about subject pronouns concepts in Portuguese (with their first person, second person, third person and their singular and plural forms). These concepts are primordial to help you to understand the conjugation rules of verbs.