Recalculate Averages based on dates

Hi everyone, this is my 1st post here and I am hoping you can help.

I am a php developer, however my employer is has asked me to 'knock something up quickly' in excel, as obviously as I know one language I can do 'all computer stuff'!

Here goes, I have attached a spreadsheet (excel 2010)

Basically we have a list of suppliers and there average delivery times in a workbook called Data.

The next workbook is basically a sales log for a member of staff. Here you can enter the order information etc. At the moment it lets us order up to 3 different shops in one order, and the spreadsheet then calculates the max delivery time based on the max average from the shops selected and adds that to the day of the order(eg we order from 2 shops, shop 1 delivers in 5 days, shop 2 delivers in 7 days, so the number of days added is 7)

Now the problem is, the data we have for average delivery times is very old, so what I want to do is update this information based on every order.

From the sheet you can see I have added 3 cells for the date each shop completes its part of the order. What I need to do is somehow work out how to recalculate the average based on this.

We have about 15 sales people, and a list of about 50 shops we order from, with a max of 4 different shops per order (only added 3 to the test spreadsheet for simplicity!)

Can anyone see what I am trying to do and point me in the right direction?

Many Thanks Kind People