I can't find any documentation on cancel offer method,

 

Do I have to grab widgets and make my own methods ?

When I did it I had to make my own

Yeah i think you can make your own

This is a little piece of code I use for canceling box 1. First Widget is to open the trade, second is the cancel button.
You'd have to change the first set of widget ID's for each box

 

if (getGrandExchange().getStatus(GrandExchange.Box.BOX_1) == PENDING_BUY) {
                            getWidgets().interact(465, 7, 2, "View Offer");
                            sleep(random(1000, 2500));
                            getWidgets().interact(465, 23, 0, "Abort offer");
                            sleep(random(1000, 2500));
                            getGrandExchange().close();
                        }