You have two options regarding VAT payment. As the IOSS scheme is optional, you can still opt to not register for the scheme and not charge the customer VAT at the point of payment.
If you choose this method and need to pay VAT, DHL will either bill your DHL account for the outstanding VAT or it will need to be paid before it can be passed through customs in the EU.
If you are registered for the IOSS or OSS scheme and the VAT has already been paid (as the VAT was paid at the point of sale), then all fees are paid, and delivery can go ahead unhindered.