google pixel 4a is $349 on most store, it seem like a pretty good deal.
About the "Phone must be activated with a Mint Mobile plan within 45 days", my personal guess is because mint mobile policy.
PLAN TERMS & CONDITIONS from Mint shows
"All service plans, including promotional plans and Trial Plans, must be activated within 45 days following the date of purchase. We reserve the right to suspend or terminate any plan or service that is not activated within 45 days following the date of purchase."