The official rule is:
- If you get a "free" trip with 100% discount (such as this bus connection) you have to use a travel day on your Flexi Pass
- If you only get a certain discount (eg 50%) you do not have to use a travel day.
Lulea - Haparanda is 155 SEK/17€, Tornio - Kemi is 6,30€ - there is no online price information for the small bit Haparanda - Tornio but it wont be much extra, maybe 10-20 SEK. Or you walk on foot if the bus ends in Haparanda - it's just some hundred metres in between the two bus terminals in SK/FI.