This sounds ridiculous but I've heard that people were able to add custom funds to your steam wallet by just using the inspect element function. Courtesy of this
Steam guide, link might be German
Right click on any of the buttons that has a value on there and select "Inspect Element" and the Chrome developer tool will open. In this guide we use $5 USD.
You will see in the developer tools a link that states: javascript:submitAddFunds( 500 );
Now the 500 value = $5, if you check $100, it'll be 10000. In this guide we'll add the 69 so it'll be: javascript:submitAddFunds( 6900 );
Once you changed the value, close the developer tool and click the button you edited.^
You will be brought to the purchase screen and thus a confirmation your custom value worked.
Purchase like usual and you'll see in the following image I have increased my wallet with a custom value of $69 USD
...or this was all just a huge troll, try it out.