File a complaint with the collection agency
Explain that the invoice is unfounded because you only ordered an introductory pack, or you did not order anything.
If the marketing or the sales talk leads you to understand that you are only offered a single sample pack or product, this means that no contract for an additional order has been concluded.
For example, sending an invoice or debiting your account is unfounded if important information concerning the order has been hidden in the terms and conditions of the contract, or you were not informed clearly about the duration of the contract during the sales call.
The collection agency has a duty to check with the company that issued the invoice that the invoice is correct and interrupt the collection process until the matter has been confirmed. The collection agency must also inform you of any possible further measures. If you receive no further information about the matter, however, contact the collection agency and ask.