Can past periods be subsequently warned in the event of a scheduled installment payment?

re
3

I bought a laptop on installments through the company of a former acquaintance.

In the purchase contract, the first working day of a month has been agreed as the payment date. Since I paid the last installment late (on 6.05) instead of 01.05 came now a reminder with 20 euro. In this reminder all past periods were noted in which I came during the payment ever in default and was justified by a letter from September in which I once received a reminder (but without due dunning fees). Now I'm unsure if I have to pay this. The seller has repeatedly behaved incorrectly towards me and I do not know how to respond.

Does anyone know about this?

Rh

The seller has several times incorrectly behave towards me

So so, …

And you do not have the purchase contract and the deadlines it contains. Is that correct behavior?

Or

It's best to react in such a way that you just pay punctually, there are such great things as a standing order because you do not forget that either.

Ka

You have been reminded once and have obviously not improved your behavior.

Therefore, the 20 euro are justified.

To what extent has the seller behaved incorrectly.

You have behaved improperly if you repeatedly paid contrary to the agreement too late.