I haven't paid for them but receive them. What should I do next?
I am not knowledgeable of Slovenia law, but it is most likely that the country follows the doctrine of contract law that the UK, many countries of the EU and America follow.
The safest, most straight-forward alternative is to ask the seller how he wants to proceed:
- he will accept the initial price (that is, 100 EUR per unit), or
- he will deposit you the amount for you to mail it back to the UK.
The ramifications of any other alternative (whether retaining the product or not asking him about the final price) are uncertain in this situation.
If you pay less than 300 EUR/unit without asking the seller, contract law could support his claim that your subsequent conduct evidences agreement as to the price of 300 EUR/unit. In other words, your decision to retain the product after getting the refund and despite knowing the corrected price of 300 EUR/unit signals your acceptance of the modified conditions.
Having mailed to you the products may likewise be interpreted as seller's subsequent conduct evidencing his eventual agreement that the price be 100 EUR/unit. However, the seller is likely to strike that position by arguing inadvertent error.
Since contract law could work either way in a situation like this, the outcome is rather random and would depend on the particular judge(s) deciding the matter. Hence the recommendation to preemptively seek consensus with the seller on how to proceed.
The worst alternative would be to retain the products without reimbursing the seller. In that case, the seller is likely to prevail in a claim of what in the U.S. is known as unjust enrichment.