To get a feel for how to increase credit balance, two different ways of transferring money to Hetzner was tried: directly from bank account (Nordea) and using the Wise service. Both were paid on Thursday and on Monday, immediately after the weekend, the credit balance had increased using both. The service charge for Wise matched exactly the amount that was left to transfer to Hetzner. The bank did not charge a service fee. The fact that both the sender and the recipient were located in the SEPA (Single Euro Payments Area) certainly helped to speed up the transaction flow. Hetzner is a German company, where its headquarters are located.
Wise's interface is not so complicated that it would need a lot of explanation. You can get off to a good start by clicking the "Send money" button. Since it seemed relevant to be able to assess the consequences of having something wrong with the transfer details, the name of the recipient (Hetzner) was set to the name of the bank and the reference number was not only set to the required Hetzner customer ID, but it also contained text string "Customer ID:", but just as quickly the payment was made available as with a bank transfer.
The payment intermediary Wise charged a service fee of 0.41 eur for a transfer of 10 eur, which is probably an acceptable amount. The amount that ends up in Hetzner's account is clearly shown and you can see from your own online bank that ten euros have been deducted from the account. The money transfer in this case has also gone through Trustly, which has the advantage that the identity is verified by a bank ID and no other proof of identity is required.
In addition to the Hetzner IBAN, it is important to include the Hetzner customer number in the reference data. A slight mistake made in the other try out allowed to deduce that the customer ID in the reference number could also contain a little extra like "Customer ID:" before the actual customer number and the amount of money would still end up where it was supposed to. However, it is probably recommended to use only the customer number as reference number. Nordea Bank did not charge a service fee (it was probably included in the personal service package).
An online bank transfer could not be much easier. Just as easy as transferring money domestically. A slightly different amount was sent for the sake of differentiation, i.e. 3 euros, which ended up in full in Hetzner's account.
PayPal's Xoom service may be as good as Wise. Other payment services that have emerged as potentially viable include Azimo, Instarem, Revolut, MoneyGram and transferGo. There are some country restrictions on the location of the sender for payment intermediaries such as Western Union, SendMoney24, GlobalWebPay and Ria. Skrill could be a good one, but at the time of writing, individuals cannot use it to send payments to businesses.
Some of these services don't let you do much until you've proved your identity in somewhat cumbersome way, possibly requiring to take a selfie, and prove your address by scanning an invoice that came in the mail, for example. In the case of the Wise service that was tried, you can use Trustly as a payment method, so you don't need to prove your identity beyond your bank ID. According to the Wise website, Trustly is available in Estonia, Finland, Spain, Poland, Sweden and Denmark.