The clothes I bought were an additional 50% off the sale price.
The clothes I purchased were half price off the already reduced price.
ーThe clothes I bought were an additional 50% off the sale price.
「買った服はセール価格から追加の50％オフだった」
additional で「追加の」
sale price で「セール価格」
ーThe clothes I purchased were half price off the already reduced price.
「買った服は、すでに割り引かれた価格から半額だった」
to purchase で「購入する」
half price で「半額」
reduced price で「割引価格」
ご参考まで！
カナダ 
