If trading-in a device whilst purchasing a new iPad, around about the time of your new iPad being despatched, you will be sent a shipping kit. The shipping kit and new device should be expected to arrive at the shipping address at about the same time - allowing time for you to transfer your data from your old to new iPad - before shipping the old iPad.
When ordering your new iPad, you will initially be charged the full price of the new iPad. When your old iPad is received, it be be assessed by Apple’s trade-in partner - and if the iPad meets the description given when initiating the trade-in - and has no unexpected damage or defects - you will receive a refund of the trade-in value directly to your credit card. If, however, your old iPad is assessed as having undeclared damage or deficiencies, you may be offered a lower trade-in value, or the iPad will be returned to you.
I hope this information proves to be helpful in addressing any concerns that you may have with the process or its anticipated timing.
Apple Trade In - Apple