@Pish_Pash replied to a similar thread not so long ago. As he quite rightly said, free postage and packaging is not really free. It is incorporated somewhat into the cost of the item. When working out profits after sale of an item, you are always taking into account delivery, whether it be £2 or £12 or even £50. Pish Pash mentioned in his post that this does in fact put you at a disadvantage.
Example
You sell a TV for £100 + £15 delivery.
You sell a TV for £115, free delivery.
To the end user, your customer, they instantly think the second item for £115 is going to be better than the first. Because it is more expensive. They do not take into consideration they are exactly the same and it's simply down to the fact one has the delivery cost incorporated into the final sale price.
Charging delivery will allow you to drop the product cost. Keeping you on top of the market, whether they offer 'free' postage.
Set up delivery weight bands, instead of delivery cost per item. I sent anything under 2kg free of charge. Larger items, 10kg for instance, £10 delivery. 10-20kg. £15 and so on.
Or, offer a discounted rate for each additional item. First item at 10kg is £10, every additional item of the same weight is £15. Instead of £20. It all depends on what you are selling when it comes down to actual delivery costs.
Over time though, I do not feel I have lost sales or gained sales from offer free or paid postage. It is a thing of the past in my honest opinion. The psychological game has gone from 'oooohhh free postage' over to 'this is better than that one because it's more'.
If you do have repeat customers, offer them free delivery for the first 2 months. But their product cost will ultimately be higher than advertised as it will have the postage built into it. They will soon come around.