I couldn’t find a published Microsoft policy on returning accessories sent with a repair, so this is the practical route I’d take.
Check the service order. Sign in at account.microsoft.com/devices, open the order, and look at the status history and any notes. If the power supply was logged as received, that’s useful evidence. Also check the replacement’s box for a charger in case one was included.
Contact Microsoft Support about that specific order. Use chat or phone from the Surface support page, and quote the service order number. Say plainly that you sent the power supply at their instruction and that it wasn’t returned or replaced. Ask them to check with the service center whether it was received and to open a case for the missing item. Get the case number.
Ask for a clear remedy. If it can’t be found, ask for a replacement power supply at no cost or a reimbursement for it. If the first agent can’t do that, ask for an escalation to a supervisor or the customer experience team.
Gather your evidence. Keep your shipping receipt and tracking number, the packing list or instructions telling you to include the charger, and any photos of the charger or box before you sent it. A record of the support instruction to include it is your strongest point.
Try another channel if support stalls. The @MicrosoftHelps account on X often gets cases escalated, and you can put the same details in writing there or by email so there’s a paper trail.