It might be good if automatic data add-on purchases could be limited to once per day, like cash balance auto top-ups are already.
I learned this today when a family member suddenly decided to use 7 GiB of data on our 1 GiB plan and caused five 1 GiB addons to be bought in quick succession. 
As a variation of this, I’ve been wondering if it would be possible to have bigger add on bundles to make it easier/cheaper like last month when I got to the end of the month and needed 2 extra 3GB bundles as I’d underestimated the amount that I’d need whilst travelling on holiday.
This is a good point & idea! Thank you 
I think this should be configurable though (perhaps the auto top-up should be as well?). Would you want to set a specific number of times, or merely once vs unlimited?
Perhaps maybe combine with a form of capping? I don’t know the finances behind the scenes, but if, say, buying two 1GB addons could automatically upgrade it to the 3GB add on (which is less expensive than 2 x 1GB) that’d be consumer friendly