In en-garde, commissions may only be purchased in Paris on the weekend. Instead of using weekend leave, can commissions be purchased using a servant or intermediary in Paris while remaining on campaign? Intermediaries can enact all weekend activities except those requiring your actual physical presence (such as proposing to your mistress). Does purchasing a commission require physical presence?
Commisons may be purchased at any time
Section K says:
After joining a regiment, a character may be promoted only if there is an opening of the next rank in the regiment. If an opening appears in a regiment, a character may purchase that rank at any time, as long as all requirements are met and the opening is still vacant.
This suggests that you absolutely can have people "take care of this for you."