Thanks all for your posts and help. It’s done. In case it helps someone in the future I write how long each step took:
Open a PostFinance USD account - 1 day
Sell the PostFinance USD fund directly into that USD account - sell order made on Friday, money arrived in USD account Monday
Transfer the USD to IB - transfer order from PostFinance to IB made on Monday, earliest execution date selectable was Tuesday.
Buy the USD ETF - money arrived in IB on Wednesday and ETF bought on Wednesday.
Transfer from PostFinance to IB cost around $18, I don’t know if that includes PostFinance’s CHF 2 fee or not.