Separate setup from upkeep
Keep one-off purchases separate from recurring bills. Note which costs are estimates and which come from your own receipts.
Costs & buying
Purchase prices, repeat orders and what it costs to keep using things.
A useful place to start.
Use the regular price
Record the price after an introductory offer, delivery fees and the order frequency. A discount on the first box is not an annual budget.
Include the replacements
Filters, inserts and spare parts belong in the comparison. So does the possibility that the thing you already own still works.
Guides & reviews
Costs & buying
Cat ownership costs in the UK: build your own budget
The cat is unlikely to contribute. A budget should probably allow for that.