Button to add a dollar amount to a field

If Table 2 is related to Table 1, you could add a lookup field to Table 1 for the [Increase Amount]. Then it would just be a matter of adding another field to add [Freight] + [Increase Amount], for your final freight amount.

I worry that if you add a button the button could be pressed multiple times and cause an incorrect amount. To avoid this would be more code that could be avoided with the above solution.

If you really want a button, my suggestion would be to still add a new field to Table 1 for the increase amount (to be populated by the button) and a total freight field. Even if the button is pressed multiple times it would only override the existing increase value vs adding to it.

URLRoot() & "db/" & [_DBID_TABLE_1] &
"?a=API_EditRecord&" &
"rid=" & [Related Record] &
"&_fid_6=[Increase amount]" &
"&rdr=" & URLEncode(URLRoot() & "db/" & Dbid() & 
"?a=dr&rid="& [Record ID#])

Replace with your actual values:  [_DBID_TABLE_1] , [Related Record],_fid_6=[Increase amount]