If a buyer has already paid for the items, I make a judgement call based on their FB history and/or the total value of the order, plus the method of postage.
If all low value items that I would have sent unregistered large letter even if purchased together, I just combine and refund any overage. Sometimes I just combine and send registered - a buyer may attempt to open an INR for other items in that package, but no seller protection doesn't necessarily mean their claim(s) will be successful.
If FB history is ok but combined order value is nearing my limit, I refund the postage excess from the lowest value item, send registered and keep that documentation to defend the highest paid item if needed (most applicable to orders with two items, really, but still workable for orders with more items).
If I won't risk it at all, for any reason, I explain that I'll need to issue a refund, send a new, combined invoice with reduced postage, and go from there (it was pre-defects last time I did that, though. Refunding will sometimes trigger one, I believe).
In my response, I tend to include instructions for checking out multiple items both for standard PCs etc and mobile apps, for their future reference, explaining that combining post-payment isn't always possible and they'll usually be able to get better combined rates if they pay for multiple items with a single payment (which is true with my items).
