honestly, when you marry, it is really not just involving 2 person, but 2 families.
even in the fufure, your life wont just revolve arnd your hubby and kids.
if your husband is using his money to buy a bag for his mom, let him be, take it as an act of filial piety.
for the catering for baby's shower, just go ahead and order then amt you want, i dont think need to inform your mother in law since you are the one paying.
unless she wants to fork out, then let her order more.