Measure columns one sum multiple columns

See if it helps

0-30 days =
SUMX (
    SUMMARIZE (
        'OINV A/R Invoice',
        'OINV A/R Invoice'[DocNum],
        'OINV A/R Invoice'[CardName],
        'OINV A/R Invoice'[DocTotal]
    ),
    CALCULATE (
        IF (
            [Invoice Age Today] <= 30,
            IF ( [AmountOwed] > 0, [AmountOwed], BLANK () ),
            BLANK ()
        )
    )
)