If you use the official Sri Lankan ETA portal (
www.eta.gov.lk) and your travel dates are within the temporary ETA free exemption period, which ends on the 30th of April 2020, you should not be charged a fee. I am a US citizen and just tried it myself, and it shows the visa fee as 0.0 USD.
There are other Sri Lankan VISA service web pages, which unfortunately all look official. These web pages might still be charging the fees. I have not tried them.
I travel to Sri Lanka at least two times a year and always use
www.eta.gov.lk, which sends an email acknowledging your ETA application and another email when the ETA is approved. However, I never print this email out and carry it with me. If you have used the correct passport number, the Sri Lankan immigration computer system gets updated with your approval and the immigration officer should see this when he or she scans your passport at the CMB airport. You said you checked your ETA approval status online. You can print this webpage and take it with you, if it make you feel more comfortable.