Excel IFS to stacked CASE WHEN

IFS is CASE with the ELSE left off. First match wins. Write the tightest test first if they overlap.

CASE
  WHEN total_amount >= 10000 THEN 'A'
  WHEN total_amount >= 1000 THEN 'B'
  ELSE 'C'
END

TRUE as a last IFS test is ELSE.

Canonical translation

CASE
  WHEN total_amount >= 10000 THEN 'A'
  WHEN total_amount >= 1000 THEN 'B'
  ELSE 'C'
END