With robust holiday retail sales comes the inevitability of a mountain of returns, and today is the biggest day for shipping firms to handle that tsunami of items. Some retailers are even easing their online  gift return policies.

Delivery company United Parcel Service says Tuesday is expected to be the busiest day of the year for returns after record e-commerce sales.  The company projects to deliver a record of over 550,000 returns on Tuesday, an increase of 8 percent from last year, according to Reuters.

UPS expects to transport over 2 million return packages total from Tuesday through Friday this week. Electronics and clothing are the most common returned items, according to Ken Burkeen, UPS marketing director of retail and consumer products.

Burkeen said the "Christmas hangover" is one reason why there may be more returns this week than last. Last week,  many people were still traveling, visiting with friends and family. Historically, Burkeen said the first business day after the holiday is when people get back to their normal routines and typically do their returns.

According to customer service rating company STELLAService, Amazon, Best Buy and Apple extended their normal return windows to accommodate the large number of returns, Reuters reported. Shoe and apparel website Zappos, a unit of Amazon, offers free shipping for sales and returns.

The National Retail Federation estimates holiday gift returns will total a record $46.3 billion in 2011, up 4 percent from the previous year.

According to commScore, people spent $35.3 billion online for the holiday season as of Dec. 28, a 15 percent increase over the same time period last year.

Consumers spent about $44 billion at retailers just in the week ending Dec. 24, a 14.8 percent gain over the same week last year, according to ShopperTrak. And the post-Christmas holiday shopping rush is expected to be just as strong.

It is best to check a retailer's return policy regarding returning online purchases in a store. Victoria's Secret now accepts the return of online purchases in its retail stores with an invoice. The return will be credited to customers after it is processed by a fulfillment center. Some items, like bathing suits, may not be returnable, however.

On Tuesday, @UPS is hosting a Twitter chat at 1 p.m. and 8 p.m. EST to learn tips for hassle-free returns. Blogger Melissa Dawn Lierman (@TimeOutMom) will moderate and the experts from @TheUPSStore will offer helpful shipping advice.