That's it for now, give my love to your father.
Write back soon.
Someone has said that it is a mistake and one should write 'That's all for now...' Is it okay to finish an informal letter this way?
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You can finish an informal letter any way you please.
If you are fussy about the layout, you might start a new line for
Give my love to your father
and possibly add something to Yours
and be sure that you wish to convey your love only to a single (living) parent (your father) rather than (both) your parents or your mother and father.
In short, there are no rules concerning ways to finish informal letters.