Jamieson, Fausset and Brown on Ecclesiastes 8:13

13. neither shall he prolong--not a contradiction to Ec 8:12. The "prolonging" of his days there is only seeming, not real. Taking into account his eternal existence, his present days, however seemingly long, are really short. God's delay (Ec 8:11) exists only in man's short-sighted view. It gives scope to the sinner to repent, or else to fill up his full measure of guilt; and so, in either case, tends to the final vindication of God's ways. It gives exercise to the faith, patience, and perseverance of saints.

shadow--(Ec 6:12; Job 8:9).