If only half your cargo is getting through, raw material might be a better bet than manufactured goods because they're more versatile? And harder for bandits to carry off. Although a fantasy world's hellish dangers are probably just going to eat the merchants anyway.

"The duke is *importing* timber? But what of the massive forests on his lands, just outside his castle walls?"

"The notorious outlaw Robin Hood has been preventing the royal lumberjacks from collecting any timber there for nigh on seven years, now..."