In traditional grammar, "shall" is used with the first person pronoun ("I" and "we") to describe a future action while "will" is used with the second ("you") and third person pronoun ("they", "it", "she", "he"). However, this rule is not being observed or even heard of nowadays: no one will batter an eyelid if you use "will" instead of "shall".
"Shall" has its place in formal instructions, commands and legal contracts. It has an element of strong assertion.
You shall indemnify us against any liability with respect to or resulting from your delay in paying or failing to pay any such GST, or any other Taxes.
You will be safe using "will keep you posted". Just as a side discussion, you can also use "will have to" to add a sense of command to a future action. Example: To accomplish my mission, I shall / will have to be very focused.
I hope this helps.