Thank you very much for supporting the game and sorry for the inconvenience. This has happened before with another customer and I suspect it may be related to the itch.io backend. I just checked a minute ago and external steam key distribution is set up and there are enough keys prepared for I Am Overburdened. Each buyer should receive one.
Could you please contact itch.io support with this issue? Since it is the holiday season, I suspect they have their hands full and may not be able to reply fast. If so and you wish to play the game using the steam client, please send me an e-mail to my mailbox with a follow-up post in this thread (indicating you've messaged me from your address, like: firstname.lastname@example.org so I know which message to look for) and I'll reply with a steam key immediately ;)