Which costs will there be if I cancel my internet at home subscription?

In the days following your termination, you will receive via e-mail a pro rata final invoice (= the normal monthly rate minus the period that your connection will not be active), or a credit note (= refund for the period that your connection will not be active) if you have already received your monthly payment invitation/invoice. No costs there. ;-) An overview of your invoices can be found here.

As for your modem and/or booster: you have to return these via bpost of course. You can find the shipping label in your My Viking account. Of course, the idea is you return everything. In case this doesn't happen, there will be a final invoice for the modem and/or booster you didn't return.