Prior to Sunday's charge, you may see an authorization charge on your account on Wednesday. This is a temporary authorization hold, not a charge. If you make changes to your account after the authorization charge is issued, it will be reflected.
Just like many other merchants, Purple Carrot uses an authorization hold to confirm your payment method. The authorization holds are removed by your card company and your card is never actually charged.
On Sunday, Purple Carrot immediately releases the hold when charging your card for the full amount.