Need to spend 3.5k in next 3 weeks to get companion voucher - idea help!

Solved - paid 3.5k to Octopus Energy which could be withdrawn later if needed or just pre pay next years worth of energy

I have money in savings to pay it straight off, but need to spend 3.5k on Amex in 3 weeks to meet the yearly amount of 15k spend

Any ideas what to spend it on? I don’t want to waste my money but thinking where could I “spend” £3.5k and just use it as credit for example…

Can’t pay my Sainsbury’s bank loan via Amex, can’t add money to childcare account via Amex…

Any ideas?