Not online merchant, but there is an option to utilise coins to offset up to 50% of payment when you choose "Show QR Code/Barcode" for payment (instead of scan the merchant's QR). The toggle appears below the "Wallet Balance" and "Use Voucher" row under the QR code. I have used this at Giant last week, probably works for any merchant that scans your displayed Shopeepay QR Code/Barcode.

PSA: cannot send Shopee coins via chat to other users anymore since v3.18.24 app update 30/1/24