Let me know when you are going to send it **so (that)** I can make the payment.

I've bought some fruits **so** you dont need to go to the grocery when you arrive

I don't understand the difference between the use of **so** and **so that**in these sentences 
 both  can answer to the question "why"

Why must you let know when you are going to send it?   to make the payment

Why have I bought some fruits  ?   to avoid   going to the grocery to you

I've bought some fruits in the intent of avoiding to go to the grocery in this sentence,   not going to the grocery  is not a result but the purpose of buying some fruits for me but I know I'm wrong 

The difference for me is not clear