They are not the same, although there is often confusion as people often refer to NSP (notional service purchase) as buying back years.
I think it’s compulsory to buy back years before NSP. I bought back years before commencing NSP.
Buying back years means making contributions for time that you have already worked, but weren’t making contributions. For example if you were on a temporary contract, before a permanent one.
NSP looks forward instead of back - if you continue working until retirement age, will you be eligible for full pension through 40 years service. If not, you can purchase NSP to bring you up to a maximum of 40 years.
I’m not an expert on this! But I understand my own circumstances well.
I can’t answer your original question though, because I’ve always paid through my payslip, rather than a lump sum.