¡Hola Joseph!
This is a great question!
Normally, abbreviations follow the capitalisation rules of the full version of the word. However, there are some exceptions, and this includes forms of address.
This means that even though when you write them out in full, usted and ustedes are all lower case, the abbreviated forms are capitalised (Ud. and Uds. respectively). Some other examples are:
- señor = Sr.
- señora = Sra.
- don = D.
Saludos
Shui
