Since your item exceeds the maximum dimensions for shop drop-off (120cm, 61cm x 46cm x 46cm), you will likely need to choose the "Custom" option and manually enter the dimensions.
Under Postage Set Up >> Delivery Details click on the > alongside Package size, scroll to the bottom you'll see:
Looking for heavier or larger package options?
If not possible on the app or mobile browser, switch if you can to a laptop /PC , use Chrome and try from there.