where do i ship my old ipad to?

You should receive a trade in kit in the mail with instructions. Text below copied from here under Timing and Process.

Apple Trade In - Apple

How do I send back my device?

We’ll either send you a prepaid trade-in kit or email you drop-off instructions. If we send you a trade-in kit, it will include instructions on how to prepare and package your device.

Here’s how to prepare and package your iPad

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Hello jbs02,

The return box should have a label (or a sticker you need to apply) that will tell you who will handle the package. This could be UPS or another company.