DigiCal is only available through the Google Play Store, so you can only buy DigiCal+ with the Google Play supported payment methods:
Credit cards, debit cards, Google Wallet balance, direct carrier billing, PayPal, Google Play gift card and promotional codes.
Not all of them are available in each country though so please see the official Google help page for which methods are available in your country:
Furthermore, for subscription based payments such as our premium weather or our calendar store; these will require either a credit card or Paypal. Google Play Store doesn't let you purchase subscriptions using gift cards and wallet balances.
*Nowadays there are also providers of prepaid credit cards and you can easily get such a prepaid card online. In the Netherlands for example, it is possible to buy Visa prepaid credit cards through:
- 3V Card: https://www.3vcard.nl