otherwise为副词，前面应该用分号或大写另起一句。除此之外这个句子没有问题。

The article will be returned intact if the girl accpts  his love；otherwise a piece of charcoal is enclosed herewith.

The article will be returned intact if the girl accpts  his love. Otherwise a piece of charcoal is enclosed herewith.

另外，charcoal为木炭，不是火炭。