Cyber Monday was on track to bring in a record $7.8 billion in U.S. online sales but will also test the limits of retailers' e-commerce operations as millions of shoppers scour for steep discounts on everything from Lego sets to big-screen TVs.

Target Corp. and Amazon.com Inc. are pulling out all the stops to get customers' attention by offering deliveries with no minimum order limits and bombarding shoppers with a slew of promotional emails.

The much-hyped marketing event is expected to bring in more than $3 billion in online, e-commerce sales volume, above the $2.4 billion logged last year, according to Mastercard SpendingPulse that measures total U.S. retail sales across all payment types.