I disagree. You have plenty of time to get both sorted out. Get the payment ready, such as putting the card/cash in your front pocket, before your turn comes up.

Then, move all the items into the cart, when the cashier is done scanning your items you whip out the card from your pocket and pay up. Then, you can place the card into your wallet while the cashier is printing the receipt (I always ask for a receipt).

The only time this can be an issue is if the cashier starts scanning your items when there next customer is still idling without moving tf out of the way, then you have the issue of catching up but the technique is still the same, it just means paying half way through the packing.