I compared a number of Slovenian domain registrars for prices for one year for a .si domain, with no hosting.
tld-registrar.com - €30 / year
101domain.com - $119 / year
slovenia-domain.com - €16.80 / year
domene.si - €19.96 / year
domovanje.si - €15.59 / year
domenar.net - €14.90
hitrost.com - €15.54
Gigaspark.com - €18.90
The cheapest of the providers that had an english option was slovenia-domain.com at 16.80 euro. They also had a paypal option, so this is the one I chose.
domenar.net also appeared to have a paypal option, but it didn't work when I tried it.
All the other providers required a bank transfer as a form of payment, and had no english option. their sales support spoke english, and were even happy to zero rate the purchase for VAT, meaning that it would have been possible to order the domain for €12.95 from HitTrost.com