Hi @Kamila Cyranska 

How much did your invoice increase?

Starting this month KPN applies an inflation correction. This means that your subscription fee increases by 6.4% or a maximum of € 4.00

This was announced by KPN one month ago on your previous invoice.

That month I had a bill of 69 e and in this 74 e and I always paid 49 e 

Did you have a look at page 2 of your invoice? There you will find the specification of the items that produce the total amount. Compare that with the previous invoice(s) and look for the differences.

Maybe you had a discount for a limited period which has finished now?