There was an error in calculating my remaining credit. I had a total of 5 credits deposited, purchased a key for 4.15 credits and ended up with 0.84 credits, instead of 0.85. Can I get this issue resolved please?

This is not a miscalculation. Any credit that is not a multiple of 0.11 cannot be converted to refined without remainder. So actually 4.15 is a float number between 4.15 and 4.16, 4.16 is a float number between 4.16 and 4.17 (and so forth). So after spending '4.15' you will end up with '0.84+', it's just normal.