Translation:Please use this soap.
"肥" in "肥皂" also means "fat" in Chinese. "皂" refers to Gleditsia sinensis not oak seeds. Ancient folks used Gleditsia sinensis seedd for laundry and the fat is the main ingredient in the more "modern" product (soap) for laundry. I think both the Japanese and Chinese versions make perfect sense in their own ways.
ps. 肥 in chinese although translates to fertilizer, the real meaning is fat, as in fertilizing the crops to make them "fatter".
The small tsu or chiisai tsu duplicates the next consonant and obviously changes pronounciation. せっけん: sekken in romaji. つかって: tsukatte. Other small letters you might find: a i ending kana(ki, shi, chi, ni, etc..) with a small y kana (ya, yu or yo) to form other sounds like きゃ(kya) or じゃ(ja), a (almost always because it might be a foreign word) katakana kana and a small vowel to form an otherwise inexisting sound with that consonant. For example: フォ: fo