Shape retention is a primary quality benchmark for flat caps. Professional buyers should look for blocked construction, where the cap is heat-pressed over a wooden or metal mold to ensure the silhouette remains consistent. Ask the supplier about their steam-setting process. For technical fit, verify that the brim (peak) is made of flexible yet sturdy PET or PE plastic rather than cardboard, which degrades when wet. Request size charts in centimeters to ensure consistency across S, M, L, and XL ranges, as headwear sizing varies significantly between Asian and Western markets.