1. edit the custom form;
2. make a new FieldGroup/subtab in the "Shipping"-tab;
3. save the custom form;
4. edit again the custom form and go to ScreenFields>Shipping, then for the "Shipping Carrier"-field, set the FieldGroup to your new created subtab;
5. go then to the FieldGroups(Shipping) and "uncheck" the "SHOW"-checkbox, for your new created subtab;
6. once you reload your SO with the customized form, you will not see the new created subtab...and the field in it as well.