Here was their response! I think it should be all good? Hopefully I do not have to do another RMA 😅

"Hi there,

There’s no need to send the PSU back, while missing items may delay this process, there should be no issues as long as this would get processed within our expected time frame. However, should there be issues with this and quite some time has passed since the last update on this, please reach out to us.

Just a gentle reminder to read the RMA instructions sent to you in the help request for when you also need to RMA again in the future. I understand that there may be some confusion, but please do reach back out to us to clarify anything you may be confused about.

Steam Support Dan"

I see that it says that now. Hopefully the RMA will make it right.

Thank you for the suggestion! I added the following to my support ticket, will update when I hear back…

Thank you for the label and return packaging guide. I dropped off my package earlier today, and was re-visiting this support ticket, and noted that it said to include the PSU cable. The email I received, however, only linked to the Return Packaging guide instructions, which said to only include the Steam Deck and the original case. I am hoping to receive a new Steam Deck Limited Edition OLED as soon as possible, and an additional PSU is absolutely not necessary. I do not want to delay this process any further. I am sorry! Do I need to send in the PSU separately?