First, it is required to differentiate between mixed and composite supply: Refer the link that I earlier shared to know more about the same.
1) If sale of 1st item is not dependent on 2nd item, then such transactions are mixed supplies. In case of mixed supplies, rate of GST for freight charge would be rate applicable for the good with highest rate of GST (in your case, GST on item no.2 is 18%). Hence rate of GST on freight charges would be @ 18%.
2) If sale of 1st item is dependent on 2nd item, then such transactions are composite supplies. In that case, rate of GST on freight charges would be rate as applicable to the principal good. Identify the principal good (item no.1 or 2) in your case and charge GST on freight charges accordingly.
3) In case only 1st item is sold on a particular day, then you may charge GST @ 5% on freight charges too.