According to CP vol 7 p. 537 'She d. 3 Sep. 1215', and ibid note (d) 'The day is given by the obituaries of Haute-Bruyère and St. Nicaise de Meulan ... She was living in 1213, when as "Amicia comitissa Leicestrie et domina Montis Fortis" she made a grant to the abbey of Les Vaux de Cernay ... but was dead in May 1216, when Simon, Count of Montfort, came to Paris ... to arrange about his inheritance, his mother Amice having died'.
The day of her death is given as 3 September in the two obituaries cited, see
https://archive.org/stream/recueildeshistor02acaduoft#page/240/mode/2up (where this is wrongly glossed as occurring after 1219) and
https://archive.org/stream/recueildeshistor02acaduoft#page/224/mode/2up. However, her anniversary was commemorated on 2 September at Joyenval abbey, see
https://archive.org/stream/recueildeshistor02acaduoft#page/300/mode/2up, on 5 September at Saint-Yved de Braine, see
https://books.google.com.au/books?id=0ob2REaMNNIC&pg=PA375 (this record appears under 4 September in the inaccurate copy made by Jacques Machaut in the 17th century) and on 10 September at Chartres cathedral, see
https://archive.org/stream/recueildeshistor02acaduoft#page/92/mode/2up.
Simon's trip to Paris in May 1216 was noted in the second continuation (written in 1220) of the chronicle by Robert of Saint-Marien d'Auxerre (that was cited in a different edition by CP), see
http://www.dmgh.de/de/fs1/object/goToPage/bsb00000864.html?pageNo=281.
This was taken from the anonymous chronicle of Laon, written by a cleric from England in 1219/20, 'Simon dominus Montisfortis, qui meruit Machabeus appellari, circa Kal. Maii venit Parisius auxilium petiturus contra Arragonenses, a quibus graves sustinebat incursus. Venit etiam de re familiari et hereditaria disponere, co quod Amicen, mater sua, concesserat infata', see
http://www.mgh-bibliothek.de/dokumente/a/a011512.pdf p. 82.
However, the year of Amice's death was not 1215 but either 1213 or 1214 - her charter for Les Vaux de Cernay dated 1213 (see
https://archive.org/stream/cartulairedelab00merlgoog#page/n223/mode/2up) naturally does not preclude her having died in September of the same year.
In any case she must have died by September in the following year, because her son Simon noted that she was deceased in a vidimus dated April 1215 reciting her charter dated 1206, 'Symon dominus Montisfortis, Dux Narbonæ, Tolosensis & Leicestrensis Comes ... vidimus quasdam litteras sigillo carissimæ dominæ & matris nostræ quondam Amiciæ Comitissæ Liecestrensis & dominæ Montisfortis ... Datum anno incarnati Verbi M. CC. sexto. Nos autem donationem istam ... roboramus. Datum anno dominicæ Incarnationis M. CC. quinto-decimo, mense Aprili', see
https://books.google.com.au/books?id=WMZhAAAAcAAJ&pg=PA330, pp. 330-331.
This is in vol 13 (1677) of Luc d'Achery's *Spicilegium*, and clearly dated 1215, not in vol 8 and dated 1216 as misstated by André Rhein in 'La seigneurie de Montfort en Iveline', *Mémoires de la Société archéologique de Rambouillet* 21 (1910) p. 62, see
http://gallica.bnf.fr/ark:/12148/bpt6k454189k/f67.image.
Peter Stewart