The short answer
Check the full upgrade transaction before buying; the fee, activation method, and device deal can matter more than the headline price.
Step by step
Open the carrier's current fee disclosure and write down any upgrade, activation, assistance, shipping, and restocking charges.
Compare self-service online checkout with an in-store or phone-assisted purchase because the fee can depend on the sales channel.
Ask whether bringing an unlocked phone, buying directly from the manufacturer, or activating through the app changes the fee.
If the fee was not disclosed or the activation failed, request a documented credit instead of assuming it will disappear automatically.
Compare the complete device and service cost over the required term before accepting a promotional upgrade.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.