Although I do believe that this is actually a feature request, I didn't think it belonged in that section because that section is typically used for larger changes, not the "spit and polish" type of request this is.
Currently, when you spend a credit in the MD shop, the number of credits remaining updates within that panel, but the number of credits you have, as listed below the MD shop button, doesn't update. This should be solvable by putting a bit of javascript in the MD shop pages to unconditionally update that number to the user's current amount, which is already displayed on most, if not all, of the pages of the shop somewhere.