Using up the yearly amount TNISA early

[Bubblegun]

My question is, for example.

Say I pay by credit card every month 15,000 yen, but I then decide to drop in 300,000 yen, or try to actually overpay, what will actually happen.

400,000 yen for a TNISA isn't a lot.
1,200,000 isn't a lot for just a NISA either.

I actually have a TNISA and would actually like to to pay more.

[TBS]

My question is, for example.

Say I pay by credit card every month 15,000 yen, but I then decide to drop in 300,000 yen, or try to actually overpay, what will actually happen.

400,000 yen for a TNISA isn't a lot.
1,200,000 isn't a lot for just a NISA either.

I actually have a TNISA and would actually like to to pay more.

If you have money available to invest, it makes sense to drop it into NISA/TNISA straight away as this will maximize the tax-free period. For TNISA use the bonus payment feature to do this.

If you try and buy more than your yearly allowance, either the transaction will be cancelled or it will be executed in a taxable account instead: e.g. https://www.rakuten-sec.co.jp/nisa/rule.html#rule05

Yep, the yearly limits could be higher. Here's hoping they go up one day