Your listings read: "PAYMENT IS REQUIRED 1-5 DAYS"
If you do not wish to combine shipment for purchases after one/two days, you may want to change your terms to avoid similar situations.
Suggesting you are willing to wait five days for payment implies a buyer could combine within that period of time.
Like you I sell "stuff" easy to mail. Personally, I will combine anything for anyone with no time limit: whatever pleases the customer is OK with me. I often have buyers who accumulate items from different listings over a two to three week period.
A bit of flexibility results in
more sales and more profit.
🙂