The question is almost never whether you have the money today. It is whether spending it today causes a problem in eleven days, and that is a harder thing to answer standing in a shop.
The useful version takes about thirty seconds and needs one number you should already have.
The thirty second version
Start with
What is genuinely available, which is your balance minus protected money minus everything committed before your next payday.
Subtract
The cost of the thing.
Divide
What remains by the number of weeks until payday.
Then ask
Whether you could live on that weekly figure. That is the actual question.
Weekly is the frame that works
A remaining balance of four hundred sounds fine and means very different things depending on whether payday is Friday or three weeks away.
Converting to a weekly figure removes that ambiguity and is the single most useful thing you can do with a spending decision, because you already have an instinct for what a normal week costs you.
Watch for the annual ones
The most common way this goes wrong is a large annual payment landing in the same period. Insurance, a subscription renewal, a tax bill.
If your available figure does not account for those, it is optimistic in exactly the months you can least afford it to be.
Instalments are not free
Splitting a payment reduces today's impact and adds a fixed commitment to the next several months, which reduces every future version of the number you just calculated.
That can be a perfectly reasonable trade. It is only a problem when the decision is made against today's balance without noticing that future months got smaller.
Where the number comes from
All of this depends on having a trustworthy available figure, which your banking app does not provide. The method for building one is in how much of your money is actually safe to spend.