It makes no difference what order they’re put in to the basket.
The customer would pay £3.05 in your example @Rozcraftz . The main postage charge is the highest one. The additional charge is used for any other items included in the order.
Like @WhimsyWooDesigns I also offer free additional postage on all my items. So if a customer of mine orders multiple items they pay postage on the item with the highest postage charge and everything else goes free. If all items are the same charge then they pay that charge once and everything else is free.
There’s a lot of leeway with weight in RMs small parcel service as it goes up to 750g or something I think. If a parcel gets big enough to take it into the medium parcel category then I use Hermes to courier it as it is far cheaper than medium parcel rates and also offers up to £50 compensation as opposed to RMs £20 … and tracking too … win win.
Even international works with weight bands so, for smaller items anyway, I often don’t have to pay any more when a customer orders multiple items. If they order enough to take their parcel to the next weight band it’s usually only a shift of £2 or £3 which is covered by the purchase of multiple items. It’s kind of like giving someone a small percentage discount for buying lots at once.
Also, it is less of a headache for me when it comes to working things out …