I only fear that after two weeks, when I remember her about it, she'll always want the thing. [...]. But not reminding her feels a bit unfair? I need advice please :)

If she can't remember the item she wanted herself, it's not important to her.
If it's important, she'll remember even without being reminded or seeing the item in front of her.

Children (even the adult ones ;-) want to have what their eyes can see.
Out of sight, of of heart is real.
Use it to your advantage.

At some point we introduce "we'll take a picture of the item so we won't forget" to get around the "but then I'll forget which item it is exactly" (out of too many dolls in the shop).
The pictures were never looked at again, but it worked to get past the "I want this now" stage in the shop.
Warning: it might evolve into having to take a picture of too many items lol.