Sum of two variables in RobotFramework

the simplest way to add two variables in robotframework without the need to call keywords: you declare it in the VARIABLES sections

*** Variables ***
${A1}               ${1}
${A2}               ${2}
${A3}               ${${A1}+${A2}}

then the output of ${A3} is: 3

By default variables are string in Robot. So your first two statements are assigning strings like "xx,yy" to your vars. Then "evaluate" just execute your statement as Python would do. So, adding your two strings with commas will produce a list:

$ python
>>> 1,2+3,4
(1, 5, 4) 

So you should use number variables using ${} and . (dots) for separator like in this example:

*** Test Cases ***
sum of variables
  ${calculatedTotalPrice} =    set variable    ${42.42}
  ${productPrice1} =    set variable    ${43.15}
  ${calculatedTotalPrice} =    Evaluate    ${calculatedTotalPrice}+${productPrice1}
  log to console  ${calculatedTotalPrice}

This will produce: $ pybot test.robot

==============================================================================
Test
==============================================================================
sum of variables                                                      ...85.57
==============================================================================