Other documents
To confirm the value of the imported goods and/or their intended usage, as well as the rate of purchase from foreign online stores, the customs broker, upon reviewing your declaration, may request some additional documents, such as an explanation. However, it will only reach the customs if the customs office requests it from the broker.
Outside of that, the customs broker can request screenshots of the online-store website containing the prices, screenshots of your correspondence with the seller, payment confirmation and similar documents confirming the declared value of your purchases.