2

Which version is better?

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

or:

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

I think so is better because the payment is the effect: you send the book, and the effect is that I must pay.

3

It expresses the purpose of the requested future action, so the expression 'so that' is more appropriate (despite colloquially, 'that' may be omitted):

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

An example where it's an effect:

You've sent it to me, so I can make the payment now.

https://www.grammar-quizzes.com/8-9.html

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0

The first option is more informal than the second option, but both are acceptable and grammatically correct - the 'that' is optional.

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