Shoppers tearing their hair out in the search for Christmas presents online amid strike-hit parcel deliveries have been urged to try a low-tech solution: their local high street.

With a quarter of annual toy sales rung up in December, the boss of The Entertainer toy chain told the Guardian it had been forced to extend its delivery window to up to seven days and drop its next-day offer due to the holdups.

“We’ve had to extend our delivery promise from three to five days out to five to seven,” said Gary Grant, The Entertainer’s executive chairman. “All the couriers are in overload because although you can still give packets and parcels to Royal Mail you can’t rely on it to deliver them.”

“We had to stop offering express delivery because we can’t guarantee it. Every day lots and lots of boxes are being delivered but not in a timely way.”

Big retail names such as Waterstones have brought forward their “last order” dates for online shoppers. The bookseller’s cut-off is now two days earlier on 19 December and its website warns the postal strike could mean a “delay to our quoted delivery times”.

About 100,000 postal workers at Royal Mail will walk out on Wednesday and Thursday in a long-running dispute over pay and conditions, threatening Christmas deliveries and leading retailers such as greetings cards company Moonpig to warn of delays.

“If the couriers are already saying they can’t cope, anybody buying from the internet expecting it to be guaranteed is now probably now running the risk of being let down,” Grant said.

“Not everything is cheaper online. You’re going to get the same price in our stores as on our website and you can pick it up and take it home. So get down the high street and experience some old-fashioned customer service.”

Richard Shorney, founder of the Shop Local campaign, said there were many tales of internet deliveries being unreliable. “Go and have a face to face experience with a human. So much nicer to meet someone rather than clicking a button,” he urged.

Marks & Spencer says most orders are being delivered within the usual timeframe but has warned some online orders “may take a little longer to arrive”. It is pushing its click and collect service, which requires no minimum spend.

David Jinks, head of consumer research at delivery price comparison site ParcelHero, said: “Planned strikes affecting traditional postal deliveries just before Christmas could throw final order dates into disarray.

“Last-minute express and next-day deliveries can cost at least a fiver extra compared to standard delivery options. Cash-strapped Brits, facing higher bills and galloping inflation, simply don’t have the money to burn on expensive delivery options.”

Retail experts are expecting shoppers to head back to high streets in greater numbers this year as fears about deliveries combine with lower fears about infection from Covid, versus last year’s Omicron-hit festive season.

Shopper numbers were still about 9% down on pre-pandemic levels last week but that compared with 11% the previous week. Traditional high streets, with a mix of eating, drinking and shopping establishments, are faring better than shopping centres according to monitoring group Springboard.

Neil Clifford, the boss of footwear retailer Kurt Geiger, said its online sales were up but its physical store sales were also up, and trading better than expected, partly because of fears about deliveries, although the store has only changed its delivery dates by one day.

He said: “I think people like shops. This is the first normal Christmas for three years and people feel good to be back out and not worried about getting ill.” He said it was not clear how the next week or so would pan out with the travel disruption, snow and cold weather competing against postal strikes to influence behaviour.

Last week Evri – which used to be known as Hermes – apologised for delays, saying: “Like everyone in the sector, we are experiencing higher than normal volumes driven, in part, by the Royal Mail strikes, as well as final-mile staffing shortages in some local areas.”

Rival firm DPD said on Monday it had suspended next-day deliveries to around 5% of UK postcodes listed on its website as a knock-on effect of the Royal Mail industrial action, adding: “Where delays do occur, we update the parcel recipient directly.”

Courier firms send regular updates to business customers at this time of year and Tom Forbes, chief customer officer at the technology firm Metapack, says a common theme is they all have local depots that are above capacity. As a result there are hundreds of postcodes around the country where deliveries are not arriving on time.

“When a depot gets overwhelmed you can’t send out the vans efficiently,” said Forbes. “You’re dealing with yesterday’s traffic as well as today’s and the only way to release the pressure is to stop the volume going into those depots. This is exactly what the carriers are doing.”

Metapack’s software connects retailers with over 350 carriers but against a picture of lower parcel numbers Forbes says couriers had not been stepping up expansion so could not readily absorb the overspill from Royal Mail. “The Royal Mail carries 40% of the parcels in the country,” he said. “No one has that type of capacity idle in a business.”

Andrew Goodacre, the chair of the British Independent Retailers Association, said: “With train strikes and postal strikes I would say the best way to secure those Christmas gifts is to visit the local shops.”