Using POA with the person's permission.
I would suggest very strongly that you make a call to the help numbers on any cards to make sure that they have you registered and any deputy.
I bring this up because one of the older relatives recently gave POA to my brother and myself.
I was able to register with the bank quite quickly and was carrying out the wishes of the person; sorting payments and accounts. I had cause to raise a query over a credit card payment and was dealing with this in conversation with the card service people.
This was ongoing when my brother registered his POA.
You can only imagine my surprise when I rang to update them on my progress or lack of only to be told that I am no longer POA and they will not talk to me.
Please be aware that this is possible. The ONLY reason I know about it is because there is an open query with the credit card.
Hope this helps someone else.