Instead of 5p, you would be better to have “free” postage. That would look like a deliberate postage pricing decision, where as 5p looks like a mistake, and might even put customers off if they worry you might spot your “mistake” and ask for more to cover postage, or be concerned that you will be out of pocket.
To offer free postage, just put £0 in both columns.
Be aware that while some customers really like “free” or low postage, others will know that they are still paying for it, but it’s hidden in the item price. For example pricing a £8 painting with free postage, is the same as pricing it at £6 with £2 postage. That’s ok if they only want to buy 1 item, but it can put people off buying multiples, as they might think they are paying for hidden postage twice (or more).
Unfortunately, we never know who will be buying and whether they are motivated by free postage, or whether they are looking to buy two or more items at the same time.
Other strategies
Not sure if you know, there is a method by which you can set the system up so you charge one amount for the first item, then a different amount for each subsequent item.
For each listing, put your normal package postage cost in the first column, then a nominal amount in the second. If a customer buys more than 1 item from you, they pay the highest postage amount, plus the smaller add-on/s for the other items.
You can also just charge a single postage charge by putting £0 in the second column of each of your listings. This is what I do.
It seems daft, but these approaches can sometimes encourage customers to add extras to their order.
Ooh, nearly forgot - just never leave the second column blank - customers will be charged full postage on every item. Put an amount in, or a zero.
UPDATE - but you do leave the other countries blank, just like you have done, if you don’t want to sell abroad. It’s a bit confusing.
Hope this makes sense 