It's a Jpn novel with the basic outline being he reincarnates as the frail son of a king and he is able to move around thanks to an amulet the king got for "free" (he promised something about the merchant's daughters). The mc also has 2 sisters, the youngest loves him while the elder sister is cold but as soon as he starts to move she becomes kinder for some reason. Oh and the plot is he is going to an academy to find a wife from the merchant daughters I think.
I've red this one. But forgot the title. I think I found it in royal road or batoto in the "I'm looking for" post in the reincarnation or transported tab The Mc has a complicated relationship w/ a goddess and she reincarnated/transferred our Mc to the said frail bodied Mc in an "hardcore difficulty" that's all I remember . but hope that it helps