Things that I have heard (I make no claim as to their veracity, consider all the following potentially misleading):

(Fairly obvious) Deliveries received is a function of the cred earned in the preceding day, your charisma and luck, as well as more esoteric factors like the number of different forums pwned and probably a random element. EDIT – this was intended to be a longer list so this wouldn’t be sat looking over-obvious at the top, but I forgot what I was going to put. So there.

(Unconfirmed) Of Charisma and Luck, one affects the number of items received, the other affects the value of those items. Can’t remember which way around it was supposed to be though.

(Probable) Pwning 4 forums a day for 3 days yields more total deliveries than 12 forms pwned on the same day.

For my part, I have Luck at 60-70 and Charisma at 45, using 4 visits generally yields 150-200K, pwning more in one day can push that up into the territory of 300-400K. Today’s deliveries were a bit above normal expectations at 480K, and that was from pwning 12 of the official forums yesterday (from the Nuclear Plant down to My Rifle is This Big)