You didn't state the rate of GST. I shall presume it's 10%. That means the sale was $3,100 and GST was $310, making a total of $3,410. When the sale was made, you

r Accounts receivable $3,410Cr Sales $3,100Cr GST payable $310Dr COGS $2,100Cr Merchandise inventory $2,100When the account is settled within the discount period,Dr Cash $3,341.80Dr GST payable $6.20Dr Sales discounts $62.00Cr Accounts receivable $3,410