OK, well this is kinda complex so stay with me.....
At the end of the row, make a column and label it sent(for example we will use column J).
In cell K1 put the following (and every K cell): =IF(J1<>"Sent",H1+20,H1)
Where H1 is the date to add 20 days to it. Be certain to change the formatting of the date cell to a date.
When you have SENT the letter, Mark it sent, and the date is not added.
I understand it may not function as you wish, but it is an excellent start at the LOGIC! Have a go, and let us know if you get stuck.
Understand, we cannot post turn key solutions. IF you wish to have one of those, post your code and where you are stuck, and we can help out!
I have said it once, I will say it again. IT!