The easiest way to run the race is to use the stripe option and I bacs a payment to your bank account. Stripe will accept payments from all over the world and you simply set the page up, share the URL and watch the sales build.
If you sell balloons by cash you can add them manually: https://www.balloon.co.uk/add-balloons-manually/ – you can add as many balloons as you want manually or by CSV upload for no extra cost. The problem you have is collecting the below information:
Balloon Name / Balloon Colour / Balloon Message / Customer email
You could try to do this manually by BACS but for small transactions of £3 (or £5) it can get complicated – a bank payment from Mr & Mrs Smith for £3 might come from an account name that’s totally different.
Another alternative is to set the balloon price to £0 and use a donation website: https://www.balloon.co.uk/donate-page/
If you were going to sell a large number of balloons you could setup you own online balloon shop and then upload the balloons by CSV: https://www.balloon.co.uk/uploading-a-csv/
Images for your shop can be downloaded here: https://www.balloon.co.uk/csv/
However – to build your own shop will take hours and you’ll need a lot of sales to justify it. If you sell 100 balloons @£3 via stripe the cost is 12% – which will be £36 and save you hours of work reconciling and chasing payments. Even using your own platform & stripe account you will still be paying 25p per transaction on a £3 sale.