First, I would like to thank you for all your kind comments. I am very glad that I did one of the first announcements of my application here. There is a very nice and active community on this forum.
Using PayPal means that everyone on Earth is able to buy the license - it is available in 195 countries and very popular. Additionally it allows to fund your PayPal account with bank transfer - which is very good because credit/debit cards which allow Internet payments are not popular in many countries.