Sometimes, even when the invoice is present on the outside of the package, customs may question the correctness of the total sum and won't just greenlight the shipment. They specifically like to do this for shipments from China/Hong Kong/Taiwan and for "suspicious" sums that are just so below the threshold where no VAT/duty is imposed. It's rarer for bigger sums but I've had that happen multiple times too already.
If that's the case you normally will get a letter in the mail asking you to come to the customs office where they have stored the package and bring the letter, photo ID, your receipt/payment confirmation, and if applicable any written communication you had with the seller. No big deal really, just annoying to drive out there and wait in line. No customs duty on poker chips, just the usual 19% VAT. You can pay there in cash or with a bank debit card (at least at my nearest office). Didn't check if they accept credit cards as well last time I was there but I'd assume yes. They will ask you to open the package in front of their eyes to verify the contents.