The bad debt line is among the fastest growing metrics at Zip Co, a 'buy now pay later' powerhouse consolidating its share of the domestic market.Debts written off more than doubled to A$12.1 million.The loss after tax at Zip - $30.3 million over the December half - was five times the loss in the same period in 2018. In an ever more crowded segment, Zip can crow about growth metrics.Active customer accounts jumped 80 per cent to 1.8 million over one year, while merchant numbers increased two-thirds to 21,000.Transaction volumes doubled to $965 million, while receivables more than doubled to $1.04 billion.Bringing on board Amazon Australia as a merchant in November headlined new merchant signings along with Big W, Kmart and Optus.