During this period, the island enjoyed increased revenues through the sale of flax, with prices peaking in 1951. However, the industry declined because of transportation costs and competition from <answer>synthetic fibres</answer>. The decision by the British Post Office to use <answer>synthetic fibres</answer> for its mailbags was a further blow, contributing to the closure of the island's flax mills in 1965.
The British Post Offices decision to use what kind of fibres hurt the flax industry?
synthetic fibres