I received a letter today (09 January) that is dated 02 January requesting a bill to be paid within 3 days. 

The bill is asking for 87.60

I paid 83.90 on December 31st last year.  Does that mean I owe 3.70?
 

The reason I transferred the above amount to o2 is because I do not receive invoices from o2, although I have a DE IBAN mandate for o2 to withdraw funds, they fail to do so.