You can't be sure if you have never bought it (via Onebip). For your doubt, yes onebip is availabe in sweden. And I think you made a little misconception with the 5$ and the 3.5$. 5$ Is the standard fee on your AGE billing site.
When you select this, it will get automatically converted to an amount of Euros that equals to this fee on your Onebip account. Assuming the phone services in Sweden are nearly the same in Belgium, I can tell you that the cost in Euros was 3.5 EUR and that is not 3.5$ extra added to the base funds. Well that was for more than a year ago. Last time I checked was that 5$ USD was 4.3 EUR and something. But that depends on the increasing and decreasing values of the currency market.
However, you are right that EU citizens have to pay more than the needed prize because when you add funds to your OneBip account, you can only add 'standard credits' with intervals of 5 EUR each. So the Availibilites for Europe are 5 EUR - 10 EUR - 15 EUR - 20 EUR at a time. If you need simply 500 AP. You will have to pay 5 EUR (and additional sms costs). The actuall fee required for your AP will get automatically substracted from this 5 EUR standard balance. And don't worry. The little excess will remain on your onebip account and will be added up the next time you add a 'standard fee' to it.
Also if you happen to pay more than the 'converted value', this is because standard operator fees apply. You still pay the standard SMS fee that your mobile carrier supplies.
On a sidenote, 5$ is the absolute cost of the 500 AP you wish to purchase, not necessarily the total cost. Additional factors that can increase your total costs are phone services. Double check your plan. Consult with your operator assistant if you think the service costs are too high.