Charging customers shipping rate twice at checkout?

Site: ysdlondon.com

So simply put, we sell a variety of home accessories as well as our wallpapers and samples on our website. These obviously vary in size and weight- for example a folded tea towel shipped in a padded shipping bag will be very different from a large melamine tray wrapped in bubble wrap and a bigger box! I set up individual shipping rates for all similarly sized/ weighted items in my store (see below for examples):

Soft envelope/ small box @ £3 UK, £5 rest of world (ROW): ie tea towel, notebook, candles
Medium box @ £5 UK, £10 ROW: ie ceramics, bento box, cup & saucer set
Large box @ £8 UK, £15 ROW: ie cushion, melamine tray

The issue is, if customers buy an item set to soft envelope (ie a tea towel) AND an item set to medium box (ie a bento box) AND an item set to large box (ie a tray), Squarespace charges them ALL shipping rates, when the reality would be that we would simply put the tea towel and the bento box inside the large box holding the tray and ship them both out together, like most people would expect- we certainly wouldn't send them out separately and charge both shipping rates!

Speaking from personal experience, if this happened to me at checkout, I would say 'Huh? Forget it' and leave! The Squarespace person I spoke to on live chat suggested offering a refund post checkout but it seems like such a faff to do, especially as I can't seem to customise the checkout page with any corresponding messages and with Paypal being our set payment processor we'd probably lose money doing this, especially since they take a percentage of the total already!

For this reason, I'm wondering if I should be setting things up with weight ranges. Like yes it might be a bit of a pain to do this and we may lose money, but it's probably a better option than people not buying anything because they're charged a wild amount of shipping, right?

Has anyone else had this issue or have any suggestions as to a workaround? Would my charge by weight idea work? If it helps, I'm in the UK and on the small business plan- no option to increase to commerce and as far as I'm aware Squarespace doesn't offer courier rates for companies outside of the US anyway....!