Avoid buying empty space by default
Pack the proposed clothing kit before deciding that 45L is necessary. If most of the load fits a smaller bag, compare the benefit of extra room with the inconvenience of a larger shape. Space for occasional purchases can be useful, but it should not obscure how the bag works on ordinary travel days.
Judge the loaded carry
A volume figure is not a comfortable-load rating. Look for the manufacturer's carrying guidance and try the backpack with a representative load where possible. Check strap adjustment, movement and access when full. For flights, compare the packed exterior with current baggage requirements rather than assuming the liter class guarantees acceptance.
