A few months ago, the UOB PRVI Miles promotion with Expedia removed one very important airline from its list of eligible participating carriers to be eligible for 6 mpd earn rate. Singapore Airlines used to be included in the list of participating carriers, but it has since been removed:
You’ll be pleased to know that if you so wish to purchase tickets on Singapore Airlines with Expedia, you can still earn 6 mpd, if you use your DBS Altitude Card here. Essentially, you earn the base rate of 3 mpd for online travel transactions, and an additional 3 mpd during the promotion period, which lasts until 31 Mar 2018. Do note the cap of S$5000 per month, and that the full amount must be charged to your DBS Altitude Card.
Full terms and conditions can be found here. Take note that the list of participating carriers may be changed from time to time without notice. It’s always best to check before you book.